Photoniques 113

N°113 

LIGHT AND APPLICATIONS I EOS & SFO JOINT ISSUE 

EXPERIMENT 

LABWORK 

BACK TO BASICS 

BUYER'S GUIDE 

SPP imaging 

Quantum entanglement 

Optical helicity 

Nanopositioner 

FOCUS ON 

OPTICAL 

FREQUENCY COMBS 

• Interferometry with optical frequency combs 

• Optical frequency combs for atomic clocks and 

continental frequency dissemination 

• Kerr frequency combs: a million ways to fit light 

pulses into tiny rings 

France/EU : 19 € Rest of the World : 25 €

Editorial 

Photoniques is published 

by the French Physical Society. 

La Société Française de Physique 

est une association loi 1901 

reconnue d’utilité publique par 

décret du 15 janvier 1881 

et déclarée en préfecture de Paris. 

https://www.sfpnet.fr/ 

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ISSN : 1629-4475, e-ISSN : 2269-8418 

www.photoniques.com 

The contents of Photoniques 

are elaborated under 

the scientific supervision 

of the French Optical Society. 

2 avenue Augustin Fresnel 

91127 Palaiseau Cedex, France 

Florence HADDOUCHE 

Secretary General Générale of the SFO 

florence.haddouche@institutoptique.fr 

Publishing Director 

Jean-Paul Duraud, General Secretary 

of the French Physical Society 

Editorial Staff 

Editor-in-Chief 

Nicolas Bonod 

nicolas.bonod@edpsciences.org 

Journal Manager 

Florence Anglézio 

florence.anglezio@edpsciences.org 

Editorial secretariat and layout 

Agence la Chamade 

https://agencelachamade.com/ 

Editorial board 

Pierre Baudoz (Observatoire de Paris), 

Marie-Begoña Lebrun - (Phasics), 

Benoît Cluzel - (Université de Bourgogne), 

Émilie Colin (Lumibird), Sara Ducci 

(Université de Paris), Céline Fiorini- 

Debuisschert (CEA), Riad Haidar (Onera), 

Patrice Le Boudec (IDIL Fibres Optiques), 

Christian Merry (Laser Components), 

François Piuzzi (Société Française de 

Physique), Marie-Claire Schanne-Klein 

(École polytechnique), Christophe 

Simon-Boisson (Thales LAS France), 

Ivan Testart (Photonics France). 

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Photonics, a Science of Precision 

and Accuracy 

The history of science teaches 

us how numerous scientific 

breakthroughs were achieved 

by increased precision and accuracy. 

It also highlights how pushing forward 

the limits of precision and accuracy 

opens the way to novel research fields 

and applications. 

The story of optical frequency combs is 

the story of an optical spectroscopy method, 

which was developed to overcome 

the limits that were reached by conventional 

methods in atomic spectroscopy. 

This technique based on phase-locked 

lasers, initiated in the 1970's, turned out 

to be a major scientific breakthrough. 

Its importance was highlighted in 2005 

when the Nobel Prize in Physics was 

awarded to J. L. Hall and T. W. Hänsch 

"for their contributions to the development 

of laser-based precision spectroscopy, 

including the optical frequency 

comb technique". Articles of this special 

feature unveil the richness of this method 

and its huge potential for pushing 

forward measurement limits in terms 

of precision and accuracy. The fast development 

of this technology also benefited 

from the remarkable work of 

photonic companies, which managed 

to implement the most advanced lasers 

and optical techniques into ergonomic 

devices providing reliable and commercial 

sources of optical frequency combs. 

Another field associated with excellence 

and precision is that of optical glass, 

whose development revolutionized optics. 

2022 was declared on May 18th 2021 

a United Nations International Year of 

Glass, and the zoom of this issue is dedicated 

to this event. Optical glass has 

deeply broadened our horizons, from 

NICOLAS BONOD 

Editor-in-Chief 

the solar system and beyond with the 

development of telescopes, to the nano/ 

micro world, in the solid state and in 

life sciences, with the constant progress 

achieved in optical microscopy. It has 

also brought communications to a new 

era by connecting billions of peoples 

with optical fibers. All these technologies 

have pushed the efficiency of 

optical glass to new standards with extremely 

precise polishing, structuring 

and transparency. The long and rich 

history of optical glass is far from being 

concluded since the soar of micro and 

nanotechnologies, a science of precision 

and accuracy, is spurring exciting 

and original concepts for tailoring light 

propagation through meta-optics. 

This international issue inaugurates the 

publication of the section "Lab work" devoted 

to original optical set-ups aimed 

at learning optics and photonics though 

experiments. And what better topic than 

the emblematic experiment on the violation 

of Bell's inequalities to inaugurate 

this section? The authors explain how 

the EPR paradox, long considered a 

“Gedankenexperiment”, has now become 

a very exciting lab work in order to 

introduce quantum concepts and related 

technologies to students. 

The preparation of this issue was 

marked by the invasion of Ukraine by 

Russia. The international scientific 

community has been deeply saddened 

by this attack on a sovereign country, 

which severely violates international 

laws. I warmly thank all the scientific 

societies that clearly and promptly 

condemned the invasion of Ukraine by 

Russia, and in particular our partners 

SFO, SFP, EOS and EDP Sciences. 

Photoniques 113 I www.photoniques.com 01

Table of contents 

www.photoniques.com N° 113 

03 NEWS 

Highlights & news 

from our 8 partners! 

NEWS 

03 SFO forewords 

04 Partner news 

14 News from the Webb 

15 Crosswords 

16 Research news 

ZOOM: INTERNATIONAL YEAR OF GLASS 

20 Optical glass: an interplay 

of challenges and success 

LABWORK 

26 Quantum entanglement in the lab 

PIONEERING EXPERIMENT 

32 Surface plasmon propagation imaging 

48 

Kerr frequency combs: 

a million ways to fit light 

pulses into tiny rings 

60 

How to select 

a nanopositioner 

solution? 

FOCUS 

Optical frequency combs 

38 Interferometry with optical frequency combs 

43 Optical frequency combs for atomic clocks 

and continental frequency dissemination 

48 Kerr frequency combs: a million ways to fit 

light pulses into tiny rings 


54 The optical helicity in a more algebraic 

approach to electromagnetism 

BUYER’S GUIDE 

60 How to select a nanopositioner solution? 

PRODUCTS 

65 New products in optics and photonics 

Advertisers 

Accumold ............................................ 39 

Aerotech .............................................. 15 

APE ........................................................ 23 

Comsol ................................................. 13 

Eksma Optics ..................................... 29 

EPIC ....................................................... 11 

Go Edmund ................................. IV e cov 

Greenfield Technology .................... 21 

HEF Photonics .................................... 55 

Imagine Optic ..................................... 27 

Intermodulation Products ..............53 

IX Blue ...................................................59 

Laser Components ........................... 35 

Le Verre Fluoré ................................... 25 

Lumibird .............................................. 57 

Menlo Systems ................................... 37 

MKS ................................................ II e cov 

Opton Laser ........................................ 63 

Pro-lite ................................................. 61 

Santec .................................................. 31 

Sedi Ati ................................................. 17 

Spectrogon ......................................... 41 

Spectros ............................................... 16 

Sutter .................................................... 47 

Tiama.................................................... 19 

Toptica ................................................. 49 

Trioptics ............................................... 33 

Wavetel ......................................... 43, 45 

Yokogawa ............................................ 51 

Image copyright (cover): ©Nathalie Picqué. 

Figure reproduced from Nature Photonics 

volume 15, pages 890–894 (2021) under a 

Creative Commons Attribution 4.0 International 

License. https://www.nature.com/articles/ 

s41566-021-00892-x

SFO forewords 

ARIEL LEVENSON 

President of the French Optical Society 

“Morally as well as physically, the first of human 

rights is the right to light.” 

“Au moral comme au physique, le premier des droits 

de l’homme, est le droit à la lumière” 

Proses philosophiques, Victor Hugo 

The United Nations International Year of Light in 

2015 was a global success in celebrating the many 

ways that light impacts society. The desire to ensure 

its legacy led UNESCO to proclaim a permanent 

annual International Day of Light, and this 

coming edition in 2022 will also be an occasion to 

remember our dear friend and colleague Costel 

Subran who passed away in January. Costel was 

a passionate organizer of International Day of 

Light activities in France, and as he would say, 

Light must go on! 

Among the many International Day of Light 

events in France, I wish to highlight the development 

of laser teaching kits by the SFO Education 

Commission intended for outreach, especially 

at secondary schools. These will be freely distributed 

to accompany dissemination projects, 

and please contact the Education Commission 

for details. Let the light penetrate everywhere! 

And...Let the sunshine in! From 4-8 July, the 

SFO Congress will take place in the Côte d'Azur. 

OPTIQUE Nice 2022 will bridge the academic 

and industrial communities, with outstanding 

plenary speakers including: Alain Aspect, Sophie 

Brasselet, Rémi Carminati, Jean Dalibard, 

Frédérique de Fornel, Jérôme Faist, Philippe 

Goldner, Aurélie Jullien, Sophia Kazamias and 

Philip Russell. The congress also includes tutorials, 

SFO Club thematic sessions, and 50 industrial 

and pedagogical exhibition stands. In addition, 

to celebrate the International Day of Light several 

events are planned: 

• an exhibition of Low Cost Innovation in Optics 

organized by the SFO Optics and Physics without 

Borders Commission, targeted to help promote 

optics in countries and regions where access to 

science and technology can be difficult 

• a workshop on gender equality organized by 

the SFO Gender Equality Commission, which 

aims to develop realistic and efficient actions 

to address this important issue 

• a Scientibus hosted by the SFO Education 

Commission and the REOD Network, which 

will showcase a range of exciting experiments 

to school students 

2022 is also the International Year of Glass, and 

Glass and Light marry well in the SFO Club for 

Guided Optics (JNOG) that recently joined with 

the Optical Fibers and Networks Club to better 

combine the academic and industrial communities 

in a reinforced JNOG. 

In this Photoniques issue celebrating Bell's 

Inequality experiments, I would like to warmly 

congratulate our colleague Alain Aspect for his 

nomination as Honorary Member of OPTICA, 

a highly deserved and prestigious recognition. 

It is a pleasure to share this international edition 

of Photoniques with the European Optical 

Society, and I would like here to affirm SFO’s full 

support of EOS’s declaration against military intervention 

in Ukraine. Our thoughts go out to 

our Ukrainian colleagues and their families, and 

all whose safety has been jeopardized. They can 

be assured of our solidarity: нехай вони будуть 

впевнені в нашій солідарності. At the same 

time, we know that our many Russian colleagues 

are equally concerned, and we look forward to 

the time in the hopefully near future where international 

science can once again show the way 

towards peace. 

Photoniquement vôtre 

Ariel Levenson 

Directeur de recherche CNRS 

Président de la SFO 


NEWS 

www.sfoptique.org 

The next Laser-Induced Breakdown 

Spectroscopy France Days, 

organized by the SFO LIBS Club 

will take place in Marseille on 

June 1 and 2, 2022 (France), on the 

Luminy campus of Aix-Marseille 

University. The LIBS 2022 Days 

will be a unique opportunity to 

bring together the entire chain 

of specialists in this increasingly 

developed instrumentation, from 

academics to manufacturers and 

instrumentation vendors. 

https://www.sfoptique.org/agenda 

THE SECOND WAVINAIRE: 

METASURFACES – JUNE 2022 

After the great success of the 

first edition, the SFO, the GDR 

Complexe and the GDR Ondes are 

pleased to announce the second 

Wavinaire – open questions which 

will be held on June. 

The wavinaires are intended for 

students, post-docs, engineers 

and researchers. This second 

edition is organized around an 

emblematic publication: 

"Light Propagation with Phase 

Discontinuities: Generalized Laws 

of Reflection and Refraction", 

N. Yu, P. Genevet, M. A. Kats, 

F. Aieta, J.-P. Tetienne, F. Capasso, 

and Z. Gaburro, Science 334, 

pp. 333-337 (2011) 

An introductory mini course on 

a basic concept necessary for 

understanding the article, as 

well as a brief presentation of its 

main results, given by a postdoctoral 

fellow, will be followed 

by a critical perspective vision 

proposed by two internationally 

recognized experts, Pierre Chavel 

from Institut d’Optique Graduate 

School and Bernard Kress 

Director, Optical Engineering 

- AR hardware - Google, who 

will discuss the industrial and 

academic impacts. The Wavinaire 

will end with questions and a 

free discussion. 

https://www.sfoptique.org/pages/ 

sfo/wavinaire.html 

WELCOME TO OPTIQUE NICE 2022 

You are cordially invited to participate in 

the 9th Congress of the French Optical 

Society SFO, which will take place in Nice, 

France from July 4 to July 8. This congress 

provides fertile ground for beneficial exchanges 

between academic and industrial 

actors of optics and photonics. 

Plenary session : Alain Aspect, Sophie 

Brasselet, Jean Dalibard, Frédérique De 

Fornel, Rémi Carminati, Jérôme Faist, 

Philippe Goldner, Sophie Kazamias, 

Aurélie Jullien and Philip Russell. 

Tutorials sessions will introduce different 

hot topics: Thermal emission at the 

nanoscale by Jean-Jacques Greffet and 

Yannick De Wilde, Single spin detection by 

Vincent Jacques, Multiphoton microscopy 

by Emmanuel Beaurepaire, Earth-space 

telemetry by Clément Courde and we will 

even go to Mars with Supercam on the Rove 

Perseverance by Pernelle Bernardi. 

OPTIQUE Nice Prizes, to promote optics 

and photonics research 

To recognize excellence, the SFO 

awards two Scientific Prizes during 

this congress: Grand Prix SFO Léon 

Brillouin and the Young researcher 

Fabry-de Gramont prize. OPTIQUE 

SFO Congress welcomes for the second 

time the Jean Jerphagnon Prize, a prestigious 

award for outstanding scientific 

contributions with high potential industrial 

impact. 

Women in Optics and Physics commission, 

to promote parity in Optics 

The congress pays a special attention to 

the number of women working in optics, 

at all responsibility levels and tends to 

parity on invited conferences. 

PhD students and young researchers 

are welcome in OPTIQUE Nice 

Our goal is to allow all PhD students to 

participate once in the congress during 

their thesis. More than 200 students are 

expected. OPTIQUE Nice 2022 will provide 

a dedicated and friendly space to 

initiate a "Youth" action… 

Nice hosts the 9 th edition of SFO biennial 

congress 

The members of local organizing committee 

orchestrated by Sébastien Tanzilli 

do their utmost efforts to accommodate 

hundreds of participants in a friendly atmosphere. 

During our networking program 

you will get to know this exciting 

city in all its aspects. 

OPTIQUE Nice 2022 is an International Day of Light event 

https://www.sfoptique.org/ 

2 ND COLLOQUIUM ON THE PHYSICS 

AND APPLICATIONS OF METASURFACES 

Fortezza da Basso, Florence, Italie, July 18-22, 2022 

This 2 nd colloquium is organized by the Nanophotonics Club of the French Optical Society (SFO) 

in collaboration with the Italian Society for Optics and Photonics (SIOF). The attendees will benefit 

from outstanding international keynote speakers, Andrea Alù (CUNY), Shanui Fan (Stanford 

University), Federico Capasso (Harvard University), Philippe Lalanne (CNRS Bordeaux), Anatoly 

Zayats (King's College London) will present the latest developments in all areas of Metasurfaces. 

https://www.sfoptique.org/pages/sfo/colloque-metasurface.html 

04 www.photoniques.com I Photoniques 113

PARTNER NEWS 

www.institutoptique.fr 

NEWS 

Research in applied optics 

at IOGS together with industrial partners 

The Institute of Optics Graduate 

School (IOGS) has been strengthening 

its applied research 

activity with industrial partners, both 

French and foreign, for a few years now, 

notably with the creation in early 2019 of 

a dedicated Industrial Photonics team at 

the Charles Fabry Laboratory (LCF, CNRS 

Joint Research Unit 8501) in Palaiseau (Ile 

de France), headed by Yvan Sortais. 

Like any team of a laboratory under 

the supervision of the CNRS, this one 

does not aim at competing with private 

offices, nor to interfere in the competition 

between companies. It responds to 

requests from industrials which contain 

at least one original and innovative 

point in terms of research, which can 

be developed in the form of a patent or 

scientific communication, and when it 

is free to do so without interfering with 

the interests of another industrial with 

whom it is already working on a similar 

or related subject. 

IOGS intends to offer the industry a response 

adapted to its needs: services, collaboration, 

supervision of doctoral theses 

(with industrial funding in particular), and 

funding applications for shared projects. 

IOGS status as a higher education and 

research institution allows industrials to 

benefit from the Research Tax Credit. 

In all cases, whether or not the initial 

request from the industrial is ultimately 

translated into a contract, technical exchanges 

are governed by a confidentiality 

agreement signed by both parties. When 

it continues beyond the initial exchanges, 

the interaction can last from a few weeks 

for the shortest services, to several years 

for research projects such as a PhD thesis. 

The Industrial Photonics team deals in 

particular with requests related to the 

design, prototyping and metrology of 

optical systems, components or surfaces, 

in particular freeform optics, or optronic 

systems, for lighting or imaging. The studies 

concern many fields (defense, automotive, 

pharmaceutical, cosmetics, etc.). 

The Industrial Photonics team works 

closely with the French Freeform Optics 

Association (Freeform Optics - Research 

and Solutions), of which IOGS is a founding 

member (see Refs. [1,2]). 

It relies on the human and material resources 

of the LCF: computational resources, 

modeling (optical, mechanical, 

photometric, thermal, thin films, radiation, 

etc.), prototyping (precision optics, mechanics, 

electronics, 3D printing, etc.), and 

metrology (deformation and roughness of 

surfaces, transmission and spectral reflection, 

radiometric and photometric fluxes, 

color and visual appearance of materials). 

More generally, IOGS offers industry the 

expertise of the teams from the three 

laboratories it oversees with the CNRS: 

the LCF, the Laboratoire de Photonique 

Numérique et Nanophotonique (LP2N, 

UMR CNRS 5298) and the Laboratoire 

Hubert Curien (UMR CNRS 5516). IOGS' 

response to industrial requests is coordinated 

by the Innovation and Industrial 

Relations Department. 

Contact: dire@institutoptique.fr. 

REFERENCES 

[1] R. Geyl, F. Houbre, Y. Cornil, Th. Lépine and Y. 

Sortais, Optiques freeform : défis et perspectives, 

Photoniques 106, p. 17 (2021). 

[2] Y. Sortais, Th. Lépine, J.-J. Greffet, Des formes 

libres dans notre champ de vision, La Recherche 

568, p. 54 (2022). 

Yvan Sortais, Lab. Charles Fabry 

Email : yvan.sortais@institutoptique.fr 

An example of a partnership study 

between the LCF and industry : Design 

and implementation of benches for 

characterizing the optical properties 

of vials for the pharmaceutical industry 

(Credit: SGD Pharma) 

CONTACT FORMATION 

CONTINUE INSTITUT D’OPTIQUE 

GRADUATE SCHOOL: 

+33 1 64 53 32 36 - +33 1 64 53 32 15 

E-mail : fc@institutoptique.fr 

www. fc.institutoptique.fr 


PARTNER NEWS 

NEWS 

www.pole-optitec.com 

AGENDA 

Vision Sttugart 

04. - 06. October 2022 

Meet us on french pavilion 

www.pole-optitec.com 

About OPTITEC 

OPTITEC's mission on the regional/national 

level is to foster and 

promote activities of the photonics 

and imaging sectors in the south 

of France and to strengthen the 

synergies between its stakeholders 

(research, higher education and 

industry sectors). 

European cluster 

At the European level, OPTITEC fosters 

the participation of its members 

in various EU research and development 

programs, notably in Horizon 

Europe. In order to enhance the 

visibility and competitiveness of its 

members it facilitates the interaction 

with the European partners. 

OPTITEC is a cluster dedicated to innovative 

technologies which harness 

light to generate, emit, detect, 

collect, transmit or amplify the flow 

of photons, from terahertz waves 

to X rays, applied to five industrial 

sectors on fast-growing markets : Security/defence 

& major scientific instruments, 

Health and life sciences, 

Industry 4.0, Smart mobility & cities 

and Digital agriculture. 

OPTITEC LAUNCHES PHASE 2 

OF ITS EUROPEAN PROJECT SUPPORTING 

EUROPEAN DUAL-USE SMES 

KETs4Dual-Use 2.0 project officially started on 16 

September 2021. The project builds on the outcomes 

of its predecessor EU KETs4Dual-Use, which 

was carried out as a Phase 1 project between September 

2018 and February 2020. 

The consortium of partners includes French clusters 

(pôles de compétitivité) OPTITEC, as the coordinator, 

Minalogic & SAFE and the defense and security related 

cluster CenSec (Denmark) as well as the Estonian 

Defence Industry Association (EDIA). 

The project intends to act as a “springboard” for 

European dual-use companies wishing to access international markets, with the 

aim of integrating sustainable long-term partnerships. The project will support 

European dual-use small and medium enterprises (SMEs) accessing markets in 

Canada, Singapore and the United Arab Emirates (UAE) and generating growth, 

notably through their participation in international missions. 

The main objective is to set up business collaboration and to stimulate demand for 

European start-ups & SMEs technologies and services in target countries to gain 

cross-border diversification and sectoral synergies representing both the supplier 

(technology/service providers) and end-user side. 

In order to efficiently support the European companies, notably SMEs, in 

their conquest of dual-use markets in target countries, various actions are 

being carried-out: 

• Setting-up of focus groups to discuss innovative solutions in the field of dual-use 

technologies and resilient business models 

• Identification of international advisors in target countries 

• Training and organisation of international missions 

• Providing SMEs with a sustainable lifecycle support following their participation 

in the missions 

In the first 6 months, the consortium has elaborated and carried out a survey acting 

as the project’s source of market needs as SMEs are required to take a mapping exercise 

to produce the data that will assist with identifying challenges and advantages 

of SMEs not only going international, but also collaborating in a cross-sectoral and 

boundary environment. 

Furthermore, a series of four workshops focusing on the role of selected KETs (cyber-security 

& artificial intelligence) for dual-use and on resilient business models is 

being organised online between March and June 2022: 

• Cyber-security - 24 March 2022 

• Artificial Intelligence (AI) - 19 May 2022 

• Access to EU funding: European Defence Fund - 2 June 2022 

• Procurement processes in target countries - 30 June 2022 

Learn more about KETs4Dual-Use project: https://clustercollaboration.eu/content/ 

european-key-enabling-technologies-dual-use-20 

LinkedIn profile: https://www.linkedin.com/showcase/eu-kets4dual-use-2-0 


PARTNER NEWS 

www.alpha-rlh.com 

NEWS 

THE ALPHA-RLH CLUSTER 

AND ITS MEMBERS AT PHOTONICS WEST 

After two years of pandemic which slowed down international exchanges, 

the ALPHA-RLH cluster accompanied its members to the 

Photonics West exhibition, one of the major photonics and laser 

events, which finally took place in San Francisco over January 25-27, 2022. 

Within the French Pavilion organised by Business France, the companies 

AA Opto-Electronic, ALPhANOV, Femto Easy, First Light Imaging, Fogale 

Nanotech, GLOphotonics and Spark Lasers had the opportunity to exhibit 

and promote their technologies. Many other members participated with their 

own booth and/or visited the trade show. PIMAP+ European project, coordinated by ALPHA-RLH, 

facilitated the participation as visitors for two members, Polytec and Mathym. ALPHA-RLH 

also met with its European partners to set-up future internationalisation activities. 

The next edition of Photonics West will be held from January 28 th to February 2 nd , 2023. 

ALPHA-RLH is planning to attend such a great event! 

NewSkin project: application 

for the second NewSkin Open Call 

ALPHA-RLH and the 33 other partners involved in the 

Horizon 2020 NewSkin project have closed the 1st open call 

on the 31st of January 2022. This was a great success as 22 

applications from SMEs, research organisations and public 

structures were selected to cooperate with the NewSkin 

partners in the framework of R&D projects. These services 

are entirely financed by the NewSkin project and will allow 

to develop and test new nanotechnological products and processes on prototypes and 

thus improve the performance of surfaces (including metals, polymers, ceramics, graphene 

etc.). A second open call will be open from April to July 2022. Do not miss this opportunity 

to join the Open Innovation Test Bed allowing to evaluate and uptake innovative surface 

nanotechnologies and connect with a unique innovation ecosystem. Register on the project 

platform to join the NewSkin community, increase your organisation’s visibility, discover 

the NewSkin service offer and apply for the NewSkin open calls. 

Contact: Romain Herault - r.herault@alpha-rlh.com 

French innovations 

at CES Las Vegas 

The CES Las Vegas, the world's 

largest event for new technologies, 

was held over January 5-7, 2022. 

A French delegation of 140 start-ups 

was present at the show. Among 

them, a delegation of 24 start-ups 

from Nouvelle-Aquitaine, supported 

by the Nouvelle-Aquitaine Regional 

Council, the French Tech Bordeaux, 

the International Chamber of 

Commerce of Nouvelle-Aquitaine 

and the ALPHA-RLH cluster. They 

had the opportunity to present their 

innovations and benefit from media 

exposure. We can mention Airudit, 

a member of the ALPHA-RLH 

cluster, specialised in the design 

of voice-controlled human/system 

interfaces, and MyEli, which won the 

CES 2022 Innovation Award for its 

connected jewel that alerts, secures 

and protects you in case of danger. 

The exhibition allowed ALPHA-RLH 

to discover the latest technological 

innovations and their applications 

on the markets, particularly in 

the fields of artificial intelligence, 

automotive technology, digital 

health, well-being and smart home. 

A great experience! 

© French Tech Bordeaux 

Available funds from PhotonHub Europe project 

to accelerate your cross-border innovation projects 

PhotonHub Europe project, whose ALPHA-RLH is one of the partners (linked third party), offers 

innovation support for European SMEs covering the entire value chain: 

• Prototyping (TRL 3-4): Highly collaborative co-innovation initiatives between the PhotonHub 

partner(s) and the company. Max grant: 100 k€. 

• Upscaling (TRL5-6): Support to optimise an initial prototype for small-series production 

in a manner which is compatible with full-scale manufacturing (possibility to include third 

parties). Max grant: 250 k€. 

• Manufacturing (TRL7-8): Brokerage service, helping companies innovating with photonics to 

rapidly connect with existing European manufacturers (2 k€ for EPIC to support the company). 

Interested companies should fill in the registration form available on the website to be 

contacted and oriented by the project’s team: https://www.photonhub.eu/application-form/ 

UPCOMING 

INTERNATIONAL 

EVENTS 

Business Mission AISTech 2022 

May 16-19 in Pittsburgh (USA) 

Manufacturing World Japan 

June 22-24 in Tokyo (Japan) 

Laser World of Photonics China 

July 13-15 in Shanghai (China) 

INPHO Venture Summit 

October 13-14 in Bordeaux 

(France) 


PARTNER NEWS 

NEWS 

www.photonics-france.com 

NEW MEMBERS 

Welcome to our new 

members : Axeclair, Cegitek 

and Luzilight ! 

BUSINESS MEETING 

Photonics for Agrifood industry 

May 24 in Paris 

The 5 th edition of Photonics Online 

Meetings, a European wide virtual 

business event dedicated to 

photonics technologies, will be held 

on 22 November 2022. 

The event will bring together major 

contractors and suppliers of photonics 

technologies and services. 

An exceptional arrangement of 

pre-scheduled and relevant meetings 

between technology suppliers and 

contractors will make this day a unique 

event during which partnerships and 

business opportunities are woven. 

In addition, a rich program of plenary 

conferences, demonstrations and 

technical presentations led by experts 

will form pattern of the event. 

For further information and to register: 

https://onlinemeetings.photonicsfrance.org/ 

AGENDA 

Laser world of photonics, 

Munich, April 26-29, 2022 

After many successful editions covering different industries, Photonics France 

organizes a Business Meeting on May 24, 2022 dedicated to the Agrifood industry. 

A full day dedicated to business and networking in Paris (Denfert- 

Rochereau), with conferences and workshops on the needs and projects of major 

contractors and integrators 

For more information and registration : www.billetweb.fr/ 

business-meeting-agroalimentaire 

The event is French speaking only; 

Call for submissions: optics and laser safety days 

JOURNÉES SECURITE OPTIQUE ET LASER (JSOL) 

November 8-9, 2022 in Bordeaux 

Business meeting agrifood, 

Paris, May 24, 2022 

[French speaking only] 

Journées securite optique 

et laser ( jsol) 

Bordeaux, November 8-9, 2022 

French photonics days 

Saint-Etienne, 

October 20-21, 2022 

[French speaking only] 

Photonics online meetings, 

Online, November 2022 

CONTACT 

PHOTONICS FRANCE 

contact@photonics-france.org / 

www.photonics-france.org 

The 4 th edition of the Optical and Laser Safety Days at Work (Journées Sécurité 

Optique et Laser au Travail – JSOL), organized by the National Optical Safety 

Committee (Comité National de Sécurité Optique - CNSO) will take place on November 

8 and 9, 2022 in Bordeaux. The objective of this event is to promote a safety and 

prevention awareness in the field of laser and optical security in companies. 

We are looking for speakers to present the risks related to the use of lasers or optical 

sources, best practices, as well as the evolution of the regulations. 

You want to submit a presentation? Send your abstract to cnso@photonics-france.org 


PARTNER NEWS 

https://nano-phot.utt.fr/ 

NEWS 

Nano-optics & Nanophotonics (NANO-PHOT) 

Graduate School 

An unparalleled 5-year program of excellence 

Training by research 

NANO-PHOT (Nano-Optics & Nanophotonics) offers an ambitious 5 years 

(Master + PhD) education program on the use of light on a nanometer scale. 

It aims at training the next generations of researchers and professionals at 

the cutting edge of nanophotonic’s sciences and technologies. 

Master program (30 ECTS credits/semester) 

The NANO-PHOT graduate School offers a rich training program that is 100 % taught 

in English. It involves more than 20 faculty members, among them 10 external specialist 

lecturers for about 15 classes. 

Semester 1 in Reims: Mathematical and numerical tools for physicists, Wave Optics, 

Solid state Physics Communication, bibliography, conferences 

Foreign Language, Lab Project I (1/day/week) 

Semester 2 in Troyes: Classical and quantum light-matter interaction; Materials and 

devices in optics and optoelectronics; Nano-optics and Nanophotonics; Microscopies 

& Spectroscopies; Nanofabrication & nanomaterials; Innovative companies: entrepreneurship, 

economic intelligence, Intellectual properties; Foreign Language; Lab 

Project II (1 day/week) 

Semester 3 in Troyes: Multi-scale characterization; Hot topics in nano-optics 

& nanophotonics; Quantum Optics and Nano-Optics; Foreign language; Management 

of research projects; Lab Project III (2 days/week) 

Semester 4: 6-month internship in a partner laboratory or company. The master 

thesis can be based on the previous Lab projects. 

PhD program (UTT doctoral school) 

• 3-years PhD research project in a Laboratory within the NANO-PHOT network 

• Joint international PhDs are possible! 

• Science and Technology Training - 60 h - examples of courses: laser use, atomic force 

microscopy, nano-optics & plasmonics 

• Employability-Focused/Human Science training - 60h - example of course: ethics and 

scientific integrity. 

Target skills 

• Basics of the multiscale interaction between light and matter 

• Knowledge and knowhow in the field of nano-optics and nano-photonics 

• Knowing the different types of optical materials and their properties 

• Knowing the behavior of light at the quantum level. 

• Skills in the use of tools of numerical simulation, nanocharacterization 

and nano-fabrication. 

• Getting aware of the potential domain of applications of nanooptics 

and nanophotonics. 

• Autonomy in a research laboratory in interaction with a research team. 

• Setting up and managing national and international research projects. 

• Understanding the economic and legal environment of innovative companies. 

Arousing students' interest in research 

begins with their involvement 

in laboratories. 

The NANO-PHOT Graduate School allows 

students to carry out "Lab projects" 

from the first year of their Master's degree. 

The NANO-PHOT students may 

have a complete access of all research 

facilities after specific trainings. Located 

on two sites, research facilities are 

spread among 7 involved laboratories 

and a 1200 m 2 platform of technology 

(including 750 m 2 of clean rooms). 

NEWS 

• A Troyes-Berlin-Ohio 

Nanophotonics Workshop 

took place in Troyes on 2022, 

February 24 th 

https://nano-phot.utt.fr/news/ 

troyes-berlin-ohio-nanophotonicsworkshop 

• NANO-PHOT has been 

sponsoring the annual French 

scanning probe microcopy 

forum that took place in the 

beautifull “baie de Somme” 

in March 2022: 

www.sondeslocales.fr/forum 2022 

• NANO-PHOT is one of the 

sponsors of the NFO16 

conference (The 16th 

International Conference 

on Near-Field Optics, 

Nanophotonics and Related 

Techniques) that will take place 

in Victoria, BC, Canada from 

29 August to 2 September 2022 

https://nfo16.ece.uvic.ca/index.html 

#clients-j 


PARTNER NEWS 

NEWS 

www.photonics-bretagne.com 

ECA-iXblue, 

a new French naval 

defense leader 

Strong attendance of the Photonics 

Bretagne network at Photonics Europe 

Groupe Gorgé, parent company of robotic 

specialist ECA-Group, has announced 

the acquisition of iXblue for 410 million 

euros. This operation will lead to a worldclass 

player in the fields of maritime, 

inertial navigation, defense, space and 

photonics. Long-standing partners, ECA 

Group and iXblue benefit from strong 

technological and commercial synergies. 

With a unique offer ranging from components 

to complex systems, the group will 

provide high performance solutions for 

critical missions in harsh environments. 

Photonics Bretagne 

expands its team 

Photonics Bretagne welcomes three 

new talents who joined the team in 

2022 to bring their complementary 

expertises to the Research and 

Technology Organisation: 

Robin POUYET 

Materials Engineer 

and Polymer Chemist 

Stéphane PERRIN 

Biophotonics 

Project Manager 

Sébastien CLAUDOT 

Technical Manager 

Photonics Bretagne and its members (IDIL Fibres Optiques, iXblue, Kerdry, Leukos, 

Lumibird, Luzilight, mirSense, Optosigma Europe, Oxxius, Silentsys) have exhibited 

at Photonics Europe in Strasbourg from April 3 to 7 and presented their latest 

innovations in sensing, imaging, lasers and related components. Photonics Bretagne has 

put in light its new range of VLMA Yb fibres, draw tower Bragg gratings, and multiple 

range of PCF (Hollow core, supercontinuum, endlessly singlemode…). 

The new Perfos® Polarisation Maintening (PM) Ytterbium doped Very Large Mode Area 

(VLMA) fibre is particularly suited for the continuously growing ultrafast fibre laser 

market. The combination of robust single mode behavior in an all-solid glass form 

factor with 750 µm 2 fundamental mode area makes this fibre an ideal tool for high-end 

industrial fibre laser manufacturers. 

The fibre Bragg gratings arrays are perfectly suited to be used as thermal or strain sensors. 

We inscribe them directly during the draw to allow the fibre to preserve its pristine 

mechanical strength. Our process allows us to inscribe in the fibre as many gratings as 

requested in a repeatable way. 

Photonics Bretagne also showed its very last development on metal coatings that allow 

optical fibres to handle harsch environment (High temperature, radiative conditions…). 

AGENDA 

Agrophotonics Webinar 

Québec-Bretagne 

May 11-12, Online 

Agrophotonics MorningTech 

“Light technologies at the 

service of animal production” 

June 9, Ploufragan (France) 

Photonics Bretagne 

General Assembly 

June 21, Lannion (France) 

5G ACCELERATION STRATEGY 

AND NETWORKS OF THE FUTURE: 

LAUNCH OF SIMBADE PROJECT 

The SIMBADE project is one of the seven initiatives selected from the French Government 

as part of the "5G Acceleration Strategy and Networks of the Future" program, and 

received funding from Direction Générale des Entreprises (DGE). The partners of the 

project are Ekinops (project leader), Idil Fibres Optiques, Le Verre Fluoré, Orange, the University 

Lille 1 - PhLAM laboratory, and Photonics Bretagne. One objective is to develop the telecom 

infrastructure and transport capabilities to unlock networks of the future. It aims to increase 

the exploitable bandwidth in DWDM transmission systems through the development of efficient 

fibre amplifiers in O-E-S telecom bands. 


PARTNER NEWS

PARTNER NEWS 

NEWS 

www.minalogic.com 

Open platform 

to explore 

new usage scenarios 

for optic sensors 

On April 5 th , together with the CEA, 

Captronic, the Auvergne-Rhône-Alpes region, 

the IRT Nanoelec and the Auvergne- 

Rhône-Alpes Entreprises agency, 

Minalogic presented at the “SystemLab” 

Regional Digital Campus an open platform 

for new imaging applications. In 

particular, STMicroelectronics, Lynred and 

Prophesee will examine with small enterprises 

new scenarios for the use of optic 

sensors by amalgamating data using artificial 

intelligence. The goal of this activity 

is to promote closer collaboration between 

potential users and the R&D actors in optical 

imaging by exploring, looking ahead to 

and opening the way for tomorrow’s technologies 

and uses. This morning event involved 

ten start-ups, small enterprises and 

micro enterprises and other organizations 

from the region. 

For more information, please go to 

the IRT Nanoelec web site or contact 

Florent Bouvier. 

AGENDA 

All eyes are on the 2022 French 

Photonics Days in Saint-Etienne 

The 4 th edition of the French Photonics Days will be held on October 20 th and 21 st , 

2022, in Saint-Etienne. The event is being hosted by Photonics France, SupOptique 

Alumni, Minalogic and Cluster Lumière, in partnership with Manutech Sleight 

Graduate School and the Institut d'Optique Graduate School, with support from 

the Saint-Étienne Urban Area and the Auvergne-Rhône-Alpes Region. 

The goal of French Photonics Days is to promote 

the French field of photonics, to give due 

credit to the regions’ actions and investments 

in this field, and to highlight the technological advances 

and market perspectives of local actors. 

The topic selected for this 4 th year’s event 

is “Photonics for Displays, Lighting and 

Manufacturing”. This topic reflects the skills and experience of the actors in the 

local networks and enables French Photonics Days to benefit from the label and 

the dynamics of the Manufacturing Biennale. 

Designed for a technical but not specialized audience, the objectives of French 

Photonics Days are to: 

• provide an update on the development of new photonics technologies and present 

their applications and markets, 

• discuss education and training, and future planning for the field of photonics, 

• promote new products at the event sponsors’ stands, 

• offer visits of local companies. 

In technical terms, the subjects adopted are: the revolution in free-form optics, 

photonics and surfaces, and new LEDs and OLEDs for lighting and displays. 

This year’s event, to be held in the UNESCO city of creative design, will also comprise 

many occasions for networking, including an evening reception. 

Registration can be done on the Minalogic and Photonics France web sites. 

Registration is free for members of the sponsoring entities (except for the reception). 

Minalogic Business Meetings 

May 31, 2022, in Grenoble 

Minalogic Annual Day, 

June 23, 2022, in Grenoble 

Laser Processing 

for Industry” talks, 

June 28-29, 2022, in Saint-Etienne 

Photonics for Health” week, 

July 4-8, 2022, in Saint-Etienne 

Vision trade fair, 

October 4-6, 2022, in Stuttgart 

“French Photonics Days”, October 

20-21, 2022, in Saint-Etienne, as part 

of the Manufacturing Biennale. 

Contact person: 

Florent Bouvier 

Photonics Manager in Minalogic 

Tel: +33 (0)6 35 03 98 52 

florent.bouvier@minalogic.com 

LASER WORLD OF PHOTONICS: 

A GREAT SUCCESS FOR THIS 2022 EVENT 

Minalogic took part in the Laser World of Photonics, the worldwide trade fair for photonics 

components, systems and applications that was held on April 26-29, in Munich. 

Our cluster accompanied a regional delegation of 9 members, under the aegis of 

its International Development Plan, with financial support from the Auvergne-Rhône-Alpes 

region. Present at the French Pavilion, operated by Business France and coordinated by 

Photonics France to bring together the French photonics field, 5 members of Minalogic (Data 

Pixel, Hef Groupe, Qiova, Set Corporation, Teem Photonics) benefited from high visibility 

and the collective strength of the groups flying the French flag. Alpao, Cedrat Technologies 

Fibercryst, Edmund Optics and Mathym were present with their own stands. 

On site, Florent Bouvier and Damien Cohen, from Minalogic’s staff, assisted the members with 

their R&D and business prospection activities. They also took advantage of the event to promote 

the regional photonics entities at the international level and to identify relevant contacts. In 

order to promote networking, conviviality and occasions for meeting people, a reception was 

held on April 27 th at the French Pavilion that brought together the members of the French photonics 

entities and a number of European partners participating in the Photonics21 platform. 

For more information, please refer to the Minalogic web site. 


PARTNER NEWS

RESEARCH NEWS 

News from the webb 

On December 25 th 2021, the largest space telescope ever built was launched from Kourou on Ariane 

5. After receiving this wonderful Christmas gift, astronomers around the world are now eagerly 

awaiting the first observations of the Webb telescope. 

After receiving this wonderful 

Christmas gift, astronomers 

around the world are now eagerly 

awaiting the first observations of 

the Webb telescope. However, unlike 

most of its predecessors, the Webb telescope 

was not optically operational 

after its launch. Six months have been 

planned to bring the Webb into its full 

scientific capabilities. As of April 1st 

2022, no big issues have arisen and 

all optical parameters that have been 

checked and tested are performing at 

the level, or above, of the expectations. 

Below is a summary of the milestones 

that have been achieved since 

the launch: 

On Jan 4 2022, the deployment of the 

five-layered sunshield that protects 

the telescope from the light and heat 

of the Sun, Earth, and Moon was successful. 

This crucial step took 7 days 

with the unfolding and tensioning of 

the five thin plastic sheets coated to 

reduce the solar power received by 

the telescope by a factor of about one 

million. The 74 cm secondary mirror 

was then deployed at 7 m in front of 

the primary mirror, which was still 

folded at that time. The telescope was 

fully deployed on January 8 when the 

two folded wings of the primary mirror 

were opened out. The Webb Space 

Telescope reached its desired orbit, 

nearly 1.5 million kilometers away 

from the Earth at the second Sun- 

Earth Lagrange point on January 24. 

While the telescope and instruments 

were still cooling down, alignment of 

the telescope started in mid-January 

and underwent a series of steps to get 

each individual segment aligned and 

cophased. 

After moving the 7 actuators of each 

segment of the primary mirror to their 

flight position, the next step was to 

identify the 18 images of a star given 

by each segments using the NIRCam 

instrument. After repositioning them 

Figure 1: Diffraction image of the same star 

by each of the 18 segments of the primary 

mirror. The segments are tilted to create a 

hexagonal grid on the NIRCam instrument. 

Each image is labelled with the corresponding 

mirror segment that captured it. Credits: NASA/ 

STScI/J. DePasquale. https://blogs.nasa.gov/ 

webb/2022/02/18/webb-team-brings-18-dotsof-starlight-into-hexagonal-formation/ 

in a hexagonal pattern (Image 1), each 

individual image diffracted by a segment 

is focused by updating the alignment 

of the secondary mirror and 

optimizing the positions of the mirror 

segments using their actuators. 

After superimposing the 18 images on 

top of each other, the segments still 

required to be co-phased with each 

other by following two steps: 

1) Dispersed fringe spectra recorded 

on NIRCam from 20 separate pairings 

of mirror segments allow a coarse 

correction of the vertical displacement 

(piston difference) between 

the segments. 

2) A fine correction is applied based 

on the estimation calculated by 

phase retrieval algorithms using defocused 

images. 

This alignment stage was completed 

on March 11 with the release of a first 

image (Image 2) showing the diffraction-limited 

image of an alignment 

star and already plenty of small galaxies 

in the background… 

Early April, this was completed with 

the alignment of other Webb instruments: 

the Fine Guidance Sensor 

(FGS), the Near-Infrared Slitless 

Spectrograph (NIRISS), and the Near- 

Infrared Spectrometer (NIRSpec). 

The Mid-Infrared Instrument (MIRI) 

continues its cooling and should soon 

reach its cryogenic operating temperature 

(7K). A second multi-instrument 

alignment will then be carried 

out to finalize adjustments of the instruments 

and mirrors. 

The instruments will be tested in their 

different observation modes, to make 

sure that they are ready for scientific 

observations. Before the end of this 

commissioning phase, a few observations 

will be made. Once processed, 

they will provide the very first images 

available to researchers and public 

illustrating the capabilities of the 

James Webb Space Telescope. Where 

Webb will look first has not yet been 

disclosed but stay tune… The images 

will probably be spectacular ! 

AUTHOR 

Pierre Baudoz, LESIA, Observatoire de Paris, 

Université PSL, CNRS, Sorbonne Université, 

Université de Paris, France 

Figure 2: First diffraction-limited NIRCAM 

image released. Hundreds of galaxies can 

already be detected in this image. © Nasa. 

https://www.nasa.gov/press-release/nasa-swebb-reaches-alignment-milestone-opticsworking-successfully. 


CROSSWORDS 

CROSSWORDS 

ON OPTICAL 


By Philippe Adam 

8 

 

10 

9 

 

1 

 

 

11 

 

2 

 

12 13 

3 

 

 

 

4 

 

5 

 

6 

 

7 

 

1 Its phase has to be compared with envelope 

2 Needed mode to generate frequency combs 

3 Interval precisely equal to the repetition rate 

4 Spanning music 

5 2005 Nobel Prize in Physics 

6 Used to measure unknown frequencies 

7 Straightforward application of FC 

8 Very straightforward application of FC 

9 Separation frequency between two modes 

10 Long wavelength today produced 

by photonic chips 

11 Nonlinear effect which could induce 

FC in micro-resonators 

12 Efficient (or finesse) factor 

13 Transmitted from pulse to pulse 

SOLUTION ON 

PHOTONIQUES.COM 


RESEARCH NEWS 

Electrically driven random lasers 

Random lasers (RLs) are intriguing devices that attract the interest of researchers for the varied phenomena 

they present in areas such as complexity, chaos etc. 

But they also have a practical aspect 

with promising applications 

as light sources for illumination 

in microscopy, sensing, super-resolution 

spectral analysis, or complex network 

engineering. RLs usually require two 

material components associated with 

the amplification (gain) and feedback 

(light diffusion) processes inherent to 

lasing. Thus, they have been customarily 

obtained from optically pumped organic 

dyes, optical fibers and crystals powders, 

or electrically pumped semiconductor 

heterostructures. Semiconductor RLs 

are usually manufactured by introducing 

light-diffusing defects in the active 

layer, something that adds a degree of 

complexity to the manufacturing process 

and spoils the ease of realization potentially 

offered by messy structures. The 

direct availability of electrically pumped 

RLs, avoiding an involved and expensive 

manufacturing process, could crush the 

principal hurdle to their use in research 

and technology. 

A team of researchers at ICMM (CSIC), 

in Madrid realized a procedure to manufacture 

a semiconductor RL from an 

off-the-shelf laser diode. The fabrication 

simply uses the high energy pulses from 

an ablating laser to sculpt the output mirror 

to create submicrometric roughness. 

The optical feedback provided by the 

ablated front mirror in combination with 

the intact rear mirror results in strong 

modification of the cavity modes and 

leads to laser emission with a random 

multimode spectrum. This in turn lowers 

the spatial coherence and messes the angular 

pattern. The speckle, characteristic 

of highly coherent light sources, is here 

strongly reduced. 

REFERENCE 

A. Consoli, N. Caselli, and C. López, 

"Electrically driven random lasing from 

a modified Fabry–Pérot laser diode," 

Nat. Photonics 16, 219–225 (2022). 

Brillouin light 

amplification in silica 

nanofiber gas cell 

Brillouin scattering in optical waveguides 

draws plenty of attentions since it has been 

widely exploited for various important applications, 

such as microwave photonics, 

highly coherent fibre laser, and distributed 

fibre sensor. EPFL scientists, in collaboration 

with FEMTO-ST Institute, have achieved 

a huge amplification of light over a 

few centimetre with tapered silica optical 

fiber surrounding by high pressure gas. The 

strong Brillouin gain in the evanescent field 

together with the high tunability offered by 

the gaz (gaz type and pression), makes the 

integrated Brillouin amplifier very distinct 

compared to its solid material counterpart. 

They research team has demonstrated a 

79-times higher peak Brillouin gain coefficient 

in the nanofibre gas cell with 57 Bar of 

C0 2 compared to that in a standard singlemode 

fibre. 

Yang, F, et al. “Large evanescently-induced 

Brillouin scattering at the surrounding of a 

nanofibre” Nat. Comm. 13, 1432 (2022). 

https://doi.org/10.1038/s41467-022-29051-8 


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BAKE-OUT, AND RADIATION APPLICATIONS 

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sealing and leak tightness requirements to withstand extreme 

environments in demanding markets such as quantum, 

space, oil & gas, electric power distribution, and nuclear. Our 

feedthroughs can achieve hermeticity where pressure is as high 

as 1000 bars, vacuum is as low as 10 -12 mbar, radiation levels 

are above 100 M Gray, temperature exceeds 200 °C or goes 

down to 0.5 K. 

BESPOKE DESIGNS TO SUIT 

MANY APPLICATIONS 

SEDI-ATI's great strength is its ability to adapt the feedthrough 

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the design of a feedthrough can be significantly 

different i.e., single or multifiber, fixed or reconfigurable, 

bulkhead or inline, with or without flange… Also, a wide 

variety of singlemode, polarization maintaining, and multimode 

optical fibers can be integrated such as specialty 

fibers, solarization resistant fibers, or metal-coated fibers. 

Finally, we devote particular attention to both the choice of 

materials and the choice of the sealing technology: epoxy, 

glass-soldering, brazing. 

ABOUT SEDI-ATI FIBRES OPTIQUES 

With 50 highly skilled and qualified professionals, a strong 

R&D team, and ISO 9001 & 13485 certifications, SEDI-ATI has 

in-house resources required to adapt and integrate an intrinsically 

fragile material such as the optical fiber to a wide variety 

of complex and extreme environments. Our essential factory 

equipment comprises two clean rooms, a pressure test bench, 

helium leak bench, two evaporation chambers, spectrometers, 

precision turning machines… 

SHARE YOUR DREAMS WITH US! 

We are always ready to listen to your needs and offer you optimal 

solutions to overcome your challenges. So do not hesitate 

to consult us! 

CONTACT 

Claire GUYONNET / Sales & Marketing Director 

contact@sedi-ati.com 

www.sedi-ati.com 


INTERVIEW 

Michael Mei, Menlo Systems 

Menlo Systems recently celebrated 

its 20 th anniversary. What were your 

initial motivations for creating a new 

company in the field of optics and photonics? 

What have been the main steps 

in terms of growth and development? 

In the 1990s, Prof. Theodor Hänsch’s 

group at the Max-Planck Institute of 

Quantum Optics performed precision 

measurements on the hydrogen atom, but 

rapidly the possibilities to measure the 

wavelengths were exhausted. Frequency 

measurement, on the other hand, was 

an almost impossible undertaking. This 

changed with the invention of the optical 

frequency comb technology: an approach 

involving the measurement of optical frequencies 

using the comb spectrum of a 

mode-locked femtosecond laser. 

In 2005, Theodor Hänsch and John Hall 

were awarded the Nobel Prize in Physics 

for their contributions to the development 

of high-precision laser spectroscopy, including 

frequency comb technology. 

Already in 2001, Theodor Hänsch, Ronald 

Holzwarth and myself had founded 

Menlo Systems GmbH with the mission 

to commercialize the frequency comb 

technology. Today, Menlo Systems is one 

of the market leaders in high-precision 

measurement technology and employs 

over 160 people worldwide. 

INNOVATION & TECHNOLOGY 

Can you describe the core of your technology? 

What are the main scientific fields 

of application of your products? 

By means of mode-locked femtosecond 

lasers, we were able to create frequency 

combs that could be used like a ruler to 

directly measure optical frequencies. 

The progress was enormous: previously, 

equipment filling a whole laboratory was 

required to measure a single optical frequency, 

whereas now we had a setup of 

approx. 1m 2 with which we could measure 

any frequency. This was a quantum 

leap for many applications. The first 

optical frequency combs were rather 

sensitive instruments, but over the years 

the technology has matured, and optical 

frequency combs nowadays are turn-key 

fiber-based laser systems designed for 

24/7 operation. 

Can you comment on the impact of 

optical frequency combs on science 

and technology? 

The impact of optical frequency combs 

on science and technology can hardly be 

overestimated. While some of the early 

adopters predicted limited applications 

in traditional high-resolution spectroscopy 

only, over the course of the last 20 

years the field of applications has widened 

and optical frequency combs are 

regarded as key enabling tools for many 

applications from optical clocks, quantum 

sensing and quantum computing, to 

detecting exoplanets in astronomy, and 

offering unmatched accuracies for tasks 

in industrial metrology. 

MARKET & STRATEGY 

Are you focusing your sales efforts in 

Europe or worldwide? 

Menlo products all have in common that 

the optimal use is in applications where 

precision counts. From the very beginning 

we had customers from all over 

the world. From our headquarters in 

Martinsried, close to Munich, we serve 

the German and most of the European 

market, supported by local experts in 

selected countries like our regional sales 

engineer located in Bordeaux. In the US, 

China, and Japan we have sales and service 

offices, with the aim to be close to the 

applications and to our customers. 

Is Europe a great place for photonics? 

Europe has a long tradition in photonics. 

Many scientific discoveries and inventions 

have been made by its excellent scientists. 

The industry benefits from the continuous 

stream of graduates out of highly-ranked 

universities. In France, there are around 

200 science labs with 13.000 scientists, and 

about 1000 companies with 50.000 jobs 

in optics and photonics (according to 

the French trade association AFOP). In 

Germany, the situation is excellent as well. 

So yes, we consider Europe a great place 

for photonics, with France and Germany 

both playing an important role. 

How do you evaluate the evolution of 

photonics? How to increase the spread 

of photonic technologies and strengthen 

the companies and market? 

With new strategic plans for quantum 

technologies in place we are observing 

the creation of several new companies 

and research projects with innovative 

quantum applications and technologies. 

In France, Menlo Systems already 

supports these initiatives by unlocking 

access to better laser metrology with 

optical frequency combs, and by providing 

customizable laser systems for demanding 

quantum applications. Menlo 

Systems will keep working closely with 

French actors of quantum technology 

to support this ambitious national plan. 

VISION & PERSPECTIVES 

How do you imagine the next 20 years for 

your company and for photonics? Have 

you identified promising scientific areas 

for photonic technologies? 

Taking the optical frequency comb as 

an example, the evolution of scientific 

areas has rapidly grown from precision 

spectroscopy to very diverse fields including 

precision metrology, optical atomic 

clocks, astronomy, space applications, 

and the quantum technologies. We are 

convinced that we currently only see the 

tip of the iceberg. By means of integrated 

optics and chip based optical frequency 

combs our vision is that advanced spectroscopic 

tools will become available with 

any smartphone in 20 years from now. 

CONTACT: Dr. Michael Mei 

Menlo Systems GmbH 

Bunsenstraße 5 

82152 Martinsried, Germany 

m.mei@menlosystems.com 


ADVERTORIAL 

TIAMA: THE GLOBAL PROVIDER 

OF REAL-TIME DATA AND QUALITY CONTROLS 

TIAMA is one of the world leaders 

in providing inspection and quality 

control solutions for the glass 

packaging industry. It offers tools for 

glassplants to enhance glass production 

process efficiency and to control the glass 

container’s integrity by detecting defects 

(visual, non-visible, dimensional, etc.). 

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offering a complete range of inspection 

solutions. Its technical expertise, innovative 

approach, and market credibility, 

enable the company to offer solutions 

that go far beyond the simple control of 

defects in glass packaging. 

In a few figures, Tiama has 91% of export 

sales, customers in 79 countries over 

the 5 continents, subsidiaries in China, 

and the USA, with teams of local technical 

engineers. Since its creation, it has 

already installed more than 10,000 online 

process and quality control solutions 

around the world. 

Tiama provides real-time data and recommendations 

to help glassmakers to 

deliver articles with the highest quality 

and to improve their productivity. To 

meet all of these needs, Tiama develops 

its expertise: 

• Intelligence: Software gathering realtime 

data across the production line 

for analysis and management of the 

plant performances with display of the 

results on a single platform for a view 

at-a-glance of the plant productivity. 

• Monitoring: The modular hot-end 

monitoring systems provide operators 

with immediate data and easily applied 

information (gob specifics, etc.) to help 

improve productivity. 

• Inspection: More than 20 years ago, 

Tiama was paving the way with the 

Atlas, a camera-based check detection, 

with automatic settings adapting themselves 

to the production changes. Since 

then, Tiama has never stopped improving 

its various machines with solutions 

that offer fast adjustments. The next 

steps, working on Big Data analysis and 

deep learning features to increase the 

predictability of glass production. 

• Traceability: Tiama has launched the 

first engraving and reading datamatrix 

code solutions and remains the only 

company offering you full unitary traceability 

of each container. 

• Service: With more than 70 local experts, 

2 training academies and a dedicated 

department focused on customer 

satisfaction, Tiama offers unrivalled 

global support services. 

Innovation 

Innovation, new products, state-of-theart, 

solutions are fully part of the Tiama 

Business model. The company holds today 

45 patents and allocates 10 % of its 

turnover to the R&D department. TIAMA 

teams have industrialized lighting systems 

and innovative image processing 

technics. Some examples: 

• Non-contact detection: following 

of collaborative project, we have developed 

a non-contact and non-destructive 

measurement system using 

microfocus X-ray source and CT technics 

in the XLAB machine. The system 

generates a full 3D image composed of 

millions of facets that can be freely rotated. 

Volume, capacity, vacuity, diameters, 

and angles can be measured as 

well as thickness fully mapped. 

• Process monitoring: for example, in 

the HOT mass systems, the cameras 

(located just under the shear cut) take 

pictures of the free-falling gobs (drops 

of fused glass) which are then analyzed. 

These data are used to measure and 

predict the trajectory of the gobs and 

regulate the gob weight automatically 

by controlling the tube and the needles 

in the feeder system. 

• Thermography: this technique consists 

in capturing infrared glass emission 

with an infrared camera to give information 

on container glass distribution. 

The system is synchronized with the individual 

section machine, so that the 

images of articles taken are linked to 

their section and cavity of origin. 

• Prediction with dynamic mask: for 

example, in the Argos system, the machine 

integrates optical devices (using 

optical fiber) but also dynamic mask allowing 

an automatic learning process 

in production, based on a reference 

mask built with production bottles. 

Conclusion 

All these projects are just a small part 

of what we develop, we strive to remain 

competitive in our field of expertise and 

always explore all our opportunities for 

innovation through collaborative projects 

within our ecosystem for example. 


ZOOM 

INTERNATIONAL year of glass 

Optical glass: 

an interplay of challenges and success 

For several hundred years, there has been an 

impressive interplay between optical design 

and optical glass development. Step by step, 

imaging by lenses in microscopes or other optical 

instruments has almost reached perfection. 

Simultaneously, optical technologies have 

broadened their application field by entering, 

e.g., the area of telecommunication and being 

even the driver of novel technologies like 

augmented reality. Remarkably, even with 

the latter dramatic change in optics, there 

has been a red thread concerning the desired 

material properties so far. However, with 

today´s nanotechnologies, also completely 

new approaches may contribute to the field of 

optical materials in future. Who knows? 

https://doi.org/10.1051/photon/202111320 

Ulrich FOTHERINGHAM * , Matthias MÜLLER 

Schott AG, Hattenbergstraße 10, Mainz, Germany 

*ulrich.fotheringham@schott.com 

Introduction to Optical Glasses 

Optical glass has a long history and has been in use for 

applications which seem far apart but are not. Who would 

imagine that the designer of glass for augmented reality has 

to cope with essentially the same issues as his predecessor 

working on glass for microscopy 100 years ago? However, 

there is a red thread connecting all this. 

Both the red thread and different applications will be presented. 

First, the "red thread properties", the transmittancy, 

the refractive index, and the chromatic dispersion will be 

introduced. Remarkably, these properties are interrelated 

due to fundamental physics – not only for glass, but for any 

optical material (Kramers-Kronig relations [1]). Often, this 

makes it difficult to simultaneously adjust these properties 

to desired values. 

Transmittancy. Preferably, optical glasses are made from 

oxides with high bandgaps so that only photons energetically 

corresponding to a frequency in the ultraviolet (UV) 

will be absorbed. These are, e.g., SiO 2 , B 2 O 3 , CaO etc. [1]. 

Low band gap, colouring impurities such as Fe 2 O 3 are carefully 

avoided. With respect to Kramers-Kronig, however, a 

desired dispersion may require oxides with medium-size 

band gaps that will move the UV absorption edge close to 

the lower end of the visible spectrum (400nm) (see Fig. 1). 

Chromatic Dispersion. Commonly, optical glasses are categorized 

in the Abbe diagram [2] according to n d , the index at 

the Fraunhofer line d (587.6nm, yellow), and a characteristic 

figure for the chromatic dispersion, the Abbe number, 

which is defined by: 

ν d = n d - 1 

— (1) 

nF - n C 

The Abbe number increases with decreasing difference 

of the indices at the Fraunhofer lines F (486.1nm, blue) 

and C (656.3nm, red), i.e. with decreasing main dispersion 

(n F – n C ). It is named after Ernst Abbe who in 1889 founded 

the Carl Zeiss foundation, the sole shareholder of the two 

companies Carl Zeiss AG and Schott AG. 

Relative Partial Dispersion. As the relation between the 

refractive index and the wavelength is nonlinear, dispersion 

characterization requires a quantity which describes the 

degree of nonlinearity. A suited one is the relative partial 

dispersion P d,C . It is the ratio between the yellow-to-red 

dispersion n d – n C (partial dispersion) and the blue-to-reddispersion 

n F – n C (main dispersion): 

P d,C = n d - n 

— C 

(2) 

nF - n C 



ZOOM 

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For a linear wavelength dependence 

of the refractive index, P d,C = 0.4 would 

hold. For real glasses, it is lower. Note 

that beside P d,C , the analogously defined, 

but differently behaving P g,F is in use. g is 

the 435.8nm Fraunhofer line. 

Refractive Index. Typical refractive index 

curves of optical glasses are given 

in Fig. 2. They are as expected from 

Kramers-Kronig [1]; one observes the 

following correlations (Figs. 1, 2): 

#1. between the refractive index and 

the UV absorption edge: the higher 

the average index in the visible spectrum, 

the closer the UV absorption 

edge to the visible spectrum; 

#2. between the UV absorption edge and 

main dispersion: the closer the UV 

absorption edge to the visible spectrum, 

the bigger the main dispersion; 

#3. between the main dispersion and relative 

partial dispersion P d,C : the bigger 

the main dispersion, the smaller 

the relative partial dispersion P d,C . 

Figure 2: Refractive 

Index in UV, VIS, NIR 

for three glasses 

from SCHOTT AG. 

Figure 1: Internal 

transmission in 

UV, VIS, NIR for 

three glasses from 

SCHOTT AG. 

The correlations #1-3 are what do make optical 

materials in the upper left of the Abbe 

diagram unfeasible! With the above background 

on optical glass, the challenges 

from optical design will now be discussed. 

Challenges from 

Conventional Optics 

and Corresponding Glasses 

Clear glass: enabler of optical imaging. 

Ca. 150 years before the first microscopes 

and telescopes were assembled in the early 

17th century, the first clear glass had 

been made by Angelo Barovier. However, 

not knowing how to remove colouring impurities, 

he titrated the melt with MnO 2 

until decolouration was reached [3]. As 

the absorption spectrum of MnO 2 is complementary 

to that of, e.g., Fe 2 O 3 , a light 

grey absorption and thus an almost clear 

appearance may be obtained – a tricky, 

but smart approach. (The first application, 

however, was tableware.) 

Greenfield Technology launches a 

new pulse and delay generator that 

provides picosecond resolution 

pulses and delays. This pulse generator 

is well suited to synchronize all 

the devices of the Picosecond Laser 

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the generator is automatically synchronized 

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receives a reference signal from the 

laser oscillator and the delay generator 

provides to all the devices of the 

system laser (Pump-laser, Q-switch, 

Pockel cell…) can receive repetitive 

or single pulses adjusted in rate, delay, 

amplitude, polarity, and width 

synchronized on the clock input with 

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This generator Model GFT1604 

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adjustable 1.5 to 5 V into 50 Ω (or 3 to 

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Optionally, pulse amplitude can be up 

to 50 V or LVDS level. Model GFT1604 

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This compact (width = 10 cm) and 

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More information on our site 

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ZOOM 


Figure 3: (left) Achromate with residual secondary colour. 

(Right) Apochromate. 

These glasses were soda/potash-lime-silicates, called "crown 

glasses" after the manufacturing method [3]. The optical position 

in the Abbe diagram is denoted by "K". 

Despite clear glass, the image quality of the early optical instruments 

was poor. Due to dispersion, "blue" rays are refracted 

stronger than "red" rays and have another focal length (primary 

colour). Different colours are imaged to different positions, 

with different magnifications, which gives rise to undesired 

colour fringes. 

Achromates: solution to primary colour. In the middle of the 

17 th century, two element lenses were introduced, with the first 

element being a converging one with strong refractive power 

(short focal length), but small main dispersion, and the second 

element being a diverging one with small refractive power, but 

strong main dispersion. 

By means of glass selection and lens element geometry, they 

are constructed such that (1) the resulting doublet is converging, 

but that (2) the strong main dispersion of the second 

element compensates the small main dispersion of the first 

element. (The dispersion by a converging element spreads colours, 

the dispersion by a diverging element recombines colours.) 

So coincidence of the "blue focus" and the "red focus" 

was reached (achromate, Fig. 3). 

Expressed as a formula, the condition for achromatism is [3]: 

1 1 

0 = — + — (3) 

f1,d ∙ν 1,d f2,d ∙ν 2,d 

The total focal length f d (referring to n = n d ) for the doublet 

follows from the additivity rule for thin lens elements: 

1 1 1 

— = — + — 

fd f1,d f2,d ∙ 

Figure 4: 

Spherical 

Aberration 

(4) 

By " 1,2 " reference to the first or second lens element is 

indicated. 

Fortunately, the high dispersion glass required had been 

invented some time before [3] by George Ravenscroft, who had 

introduced lead silicate for tableware, with its sparkling due to 

high chromatic dispersion being a very much desired feature. 

Consider, for instance, PbO·2SiO 2 . Its bandgap is 3.1 eV which 

corresponds to 400nm [3]. With correlation #2 (the closer the 

UV absorption edge to the visible, the bigger the main dispersion), 

a high dispersion glass can be expected. Such low Abbe 

number glasses are called "flints". Two names are attached to 

the first achromates: Chester Moore Hall (made the invention), 

and John Dolland (got the patent and made the money) [3]. 

Today, lead-free alternatives are often preferred. So beside 

the lead silicates with the acronym "SF", the lead- and arsenic-free 

"N-SF"-glasses, situated at the same positions in the 

upper right of the Abbe diagram, are available. 

Figure 5: Lightguide 

Apochromates: solution to residual secondary colour. In 

Fig. 3 (right), the yellow focal length is slightly smaller than 

that for blue and red. This is due to the nonlinear wavelength 

dependence of the refractive index, because of which 

the blue-to-yellow-dispersion is much bigger than the yellow-to-red-dispersion. 

If the spreading of wavelengths caused 

by the converging element is to be compensated by the diverging 

element, the ratio of the blue-to-yellow-dispersion to the 

yellow-to-red-dispersion must be the same for both elements. 

This is equivalent to equal relative partial dispersions. 

However, correlation #3 says that the bigger the main dispersion, 

the smaller the relative partial dispersion P d,C . is. So 

even if the main dispersion of the second element is strong 

enough to compensate the main dispersion of the first element, 

the partial dispersion will not be. This is why the yellow focal 

length is smaller than the one of red and blue. Therefore an 

apochromate with all colours in one focus did not yet exist 

when in the late 19 th century, Otto Schott appeared on the scene. 

Otto Schott could not override Kramers-Kronig. What he 

could do was to slightly scale up the index curve by increasing 

packing density. Thus, the average index and the main dispersion 

in the visible could be increased with neither moving the 

UV absorption edge nor affecting relative partial dispersion. 

For this purpose, he switched from silicates to borates. 

PbO·2B 2 O 3 , for instance, has a bigger band gap and a higher 

packing density than PbO·2SiO 2 [3]. Starting with PbO·2B 2 O 3 , he 

developed the borate flint S7 [3] (n D = 1.61, ν D = 44.3) that, with 



ZOOM 

Figure 6: Three 

layers of glass 

in a waveguide 

for augmented 

reality (AR) 

glasses. 

Taken with 

permission 

from [6]. 

respect to relative partial dispersion, perfectly 

fitted to the high-potassium crown 

glass O13 [3] (n D = 1.52, ν D = 58). This and 

other "special short flints" enabled the 

first apochromates, Fig. 3 (right). (In Otto 

Schott´s literature, the Fraunhofer line d 

is replaced with the nearby line D.) 

First, chemical resistivity was an issue 

both for the borate flints and the high 

potassium crown glass. One approach 

by Otto Schott to settle this was to mix 

the two glass formers B 2 O 3 and SiO 2 , a 

pioneering step in glass development. 

Today, for example, the special short flint 

N-KZFS11 and the crown glass SCHOTT 

N-BK7® are borosilicates – such as numerous 

other optical and technical glasses. 

Spherical Aberration: suppression requires 

high indices. Lens elements can 

easily be ground and polished by simple 

rotating devices [1], however, only spherical 

surfaces can thus be made. With 

them, the focus depends on the distance 

between rays and optical axis (longitudinal 

spherical aberration, Fig. 4). A second 

order calculation gives [1]: 

f = — R – — h2 

(5) 

n-1 2R 

So to suppress spherical aberration 

while maintaining f, a simultaneous increase 

of R and n is necessary. Due to 

Kramers-Kronig (correlation #1), this is 

especially challenging for low dispersion 

glasses. 

Again Otto Schott was to find the solution, 

being the first to melt barium-containing 

glasses in an optical quality. With a 

band gap of 6eV, much higher than the one 

of PbO, BaO is well suited for low dispersion 

glasses. Although Kramers-Kronig 

prevents an ultra-high index, a moderately 

high index is achieved by the high 

packing density of, e.g., glassy sanbornite 

BaO·2SiO 2 . Thus the barium crowns "BAK" 

as well as the (very) dense crowns "(S)SK" 

were launched. A later milestone 

Figure 7: FOV in AR glasses with different indices. Left: FOV with binocular 

vision. Centre: FOV with display glass, n = 1.5. Right: FOV for AR glass with n = 

1.6 (smallest FOV) up to n = 2 (largest FOV). Taken with permission from [6]. 


ZOOM 


concerning high index and low dispersion was lanthanum glass 

(George W. Morey, [1]). 

Today´s high-end microscopes such as the ZEISS Axio 

Observer® offer sophisticated features like artificial-intelligence-based 

sample finders for biological samples to prevent 

them from the lasting illumination during a lengthy sample 

inspection by eye. The optical core is a high-end apochromate 

– with its roots going back to the extremely fruitful cooperation 

of Ernst Abbe, Carl Zeiss, and Otto Schott. 

Novel Optics 

Figure 8: 

Cross section 

of a photonic 

crystal fibre 

manufactured 

by Schott AG. 

"Lightguiding": key to the photonic revolution. Out of the 

numerous recent optical inventions, the most striking glass-related 

ones exploit the phenomenon of lightguiding. Already 

discovered in the 17th century by Johannes Kepler [4], total 

internal reflection had to wait until the 20th century with its first 

commercial use for illumination, imaging and communication. 

Transmission is crucial. In conventional optics, light has 

to pass through centimeters of material only. Glass fibres for 

telecommunication, on the contrary, have to be so free of impurities 

that one could look through a several kilometer thick 

plate made of that glass. Manufacturing is by chemical vapour 

deposition [5]. Index and dispersion also play an important role. 

The acceptance angle of a lightguide, i.e. the maximum angle 

that is compatible with total reflection, is given by 

sin(α) = √ — n 12 - n 2 

2 

(5) 

where n 1 is the core index and n 2 is the cladding index (see 

Fig. 5). 

The desired indices depend on the application. For telecommunication, 

a low acceptance angle is preferred. Light entering 

at different angles would take different paths and thus cause 

an unwanted broadening of optical pulses. So no high indices 

are needed. Core and cladding are usually made from vitreous 

silica with certain dopants [5]. 

It is different for lightguiding in augmented reality (AR, [6]), 

Fig. 6. Here, the acceptance angle determines the field of view 

(FOV, [6]). The higher the index, the broader the field of view is. 

So high index glasses are required. Dispersion management is 

by different layers for different colours. 

Photonic Crystals and Meta-Materials: exploiting the wave 

nature of light. Although the principles of lightguiding can be 

explained with ray optics, the complex behaviour of, e.g., telecommunication 

fibres can only be understood considering the 

guided light as propagating modes, i.e. hybrid electromagnetic 

waves which are propagating along the fibre axis and standing 

waves in the perpendicular plane. 

The wave character becomes particularly important if the 

confinement of light to the core is caused by interference in the 

cladding. That may occur at structures with a crystal-like order, 

so-called photonic crystals (Fig. 8) or at disordered structures, 

as it is the case for lightguiding with Anderson-localization 

where lightguiding is due to the interference of scattered light 

with the scatter centres lying closely together [7]. 

Photonic crystal fibres offer many different features, among 

them being special dispersion management for telecommunication, 

low-loss transport of laser power, and others [7]. 

Nevertheless, photonic crystal fibres are only a small part 

of what is generally possible with photonic crystals, i.e. ordered 

structures where periodicities with the magnitude of the 

wavelength cause special effects. Even more striking effects 

are possible with sub-wavelength structures with which very 

peculiar phenomena may be generated, e.g., wave propagation 

and energy transport in opposite directions which is equivalent 

to a negative refractive index [7]. With respect to the highly 

sophisticated nanostructures involved, such materials will 

certainly not replace classical optical materials, but will offer 

highly welcome add-ons of the toolbox. 

Conclusion 

It is a long way from classical optical imaging to today´s optical 

applications like augmented reality. Nevertheless, glassmakers 

are all times confronted with similar issues concerning 

transmission, refractive index, and dispersion. This is due to 

the fact that said properties are inter-related by fundamental 

physical laws which prevent "once and for all times" solutions. 

On the other hand, these constraints are providing an ever-lasting 

challenge to find the limits of feasibility, employing all 

modern technologies like, e.g., nanostructuring. 

REFERENCES 

[1] U. Fotheringham et al., Optical Glass: Challenges From 

Optical Design, in: Encyclopedia of Materials: Technical 

Ceramics and Glasses, M. Pomeroy Editor-in-Chief, (Elsevier, 

Amsterdam, 2021) 

[2] https://www.schott.com/de-de/products/ 

optical-glass-p1000267/downloads 

[3] U. Fotheringham, Opt. Mat. Express Feature Issue 

Celebrating Optical Glass - IYOG 2022, in press 

[4] Molecular Expressions: Science, Optics and You - Timeline - 

Johannes Kepler (fsu.edu) 

[5] U. Fotheringham, Glastechn. Ber. 62, 52-55(1989) 

[6] R. Sprengard, Inf. Disp. 36, 30-33 (2020) 

[7] Encyclopedia of Modern Optics, 2 nd edn., B.D. Guenther, 

D.G. Steel eds., Elsevier Amsterdam (2018). 


ADVERTORIAL 

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KEY TECHNOLOGIES OF THE 21 ST CENTURY 

Our company 

Since its foundation in 1977, after the discovery of fluoride 

glasses by Poulain Brothers at Rennes University in 1974, Le 

Verre Fluoré (LVF) has been thriving in the world of fluoride 

glass technology as an expert and main innovator in this field. 

Among LVF outstanding pioneering achievements, we can 

cite the first commercial fluorozirconate glass fibers (1983), the 

first single mode fluoride fibers (1988), the first ZBLAN fiber 

lasers (1989) and the first fluoroindate glass fibers (1992). 

Over the years, LVF has developed unique manufacturing 

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LVF is today the world leading fluoride and germanate glass 

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Why LVF fluoride glass fibers 

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LABWORK 

Quantum entanglement in the lab : 

an experimental training platform 

for the second quantum revolution 

/////////////////////////////////////////////////////////////////////////////////////////////////// 

Benjamin VEST 1 , Lionel JACUBOWIEZ 1 

1Université Paris-Saclay, Institut d’Optique Graduate School, CNRS, Laboratoire Charles Fabry, 91127 Palaiseau, France 

*lionel.jacubowiez@institutoptique.fr, benjamin.vest@institutoptique.fr 


This is an Open Access article distributed under the terms of the 

Creative Commons Attribution License (http://creativecommons.org/ 

licenses/by/4.0), which permits unrestricted use, distribution, and 

reproduction in any medium, provided the original work is properly 

cited. 

The recent and rapid progress in the field of quantum 

technologies stimulates developments of specific 

courses and experimental training for future engineers 

and scientists. We describe below the experimental 

setup developed at Institut d’Optique Graduate 

School for engineering and Master students. During 

a labwork session afternoon, students can study a 

source of polarization entangled state pairs of photons 

and perform an experimental violation of Bell’s 

inequalities. This emblematic experiment is one of 

the experiments dedicated to quantum photonics. 

It was built in 2005 in the LEnsE (the Laboratoire 

d’Enseignement Experimental) of the Institut d’Optique 

Graduate School and has been perfectly working for 

almost eighteen years already. 

Entangled states represent 

one of the most strikingly 

counter-intuitive features of 

quantum mechanics, and as such, 

are often held as a great example 

of what quantum world means. 

Before the experiments by Alain 

Aspect and his team at Institut 

d’Optique reporting Bell’s inequalities 

violation in the early 80s [1], 

decades of research have led to 

tremendous results revolutionizing 

our understanding of light-matter 

interaction. The first revolution of 

quantum physics has deep connections 

with the wave-particle-duality 

and the description of physical 

systems using the concept of wave 

functions. For instance, the process 

of carrier photogeneration 

by semiconducting devices is a direct 

consequence of the ability of 

quantum mechanics to explain the 

structure of matter and the optical 

properties of materials. As a consequence, 

it is reasonable to consider 

that many experimental labwork 

sessions in the LEnsE, dedicated 

to cameras and light detectors, are 

true quantum experiments! 

However, the term “quantum photonics” 

usually refers to the concepts 

that emerged and were popularized 

after a second quantum revolution. 

Starting from the Aspect 

experiments, this second quantum 

revolution is focused on the most 

surprising and counter-intuitive 

predictions of quantum mechanics 

whose manipulation can 

lead to the development of a new 

generation of sensors, quantum 


LABWORK 

communication schemes, simulators 

or quantum computing. 

A two-photon polarization 

entangled state 

Entangled states are a class of multi-particle 

states whose existence 

is predicted by quantum mechanics 

[2,3]. The state of the polarization 

entangled pairs of photons that 

we produce in the labwork experiment 

can be written as: 

|ψ 2ph = 1 — √ 

— 2 

(|V I |V II + |H I |H II ) 

This is a typical polarization entangled 

state where V and H refer to the vertical 

and horizontal direction of polarization 

and I and II refer to each photon 

of the pair. 

Why this entangled polarization state 

is so extraordinary? So strikingly 

counter-intuitive? 

To explore more in details the features 

exhibited by this two-photon entangled 

state in polarization, we will perform 

measurements on the polarization of 

each photon of the pair. Figure 1 below 

describes the classical polarization 

measurement setup that we use in 

our experiment. 

For the entangled state |ψ 2ph , whatever 

the direction of polarization, α , photon I is 

detected 50% of the time in state |V I α and 

is detected 50% of the time in state |H I α 

(the probabilities are : P (|V I α ) = P (|H I α ) 

= 1/2). The same result would be obtained 

for photon II (P (|V I β ) = P (|H I β ) 

= 1/2, whatever the direction of projection 

β is). So, in this entangled state, none 

of the photons of the pair has initially a 

defined polarization state and the measurement 

process attributes randomly 

the state |V I α or |H I α to the photon I. 

A question now arises: what does 

happen with the other photon of the 

pair, let us say photon II? The answer 

is hard to believe: it is not even necessary 

to measure the state of photon 

II! Quantum formalism tells us 

that the polarization state of photon 

II is identical to the polarization state 

already measured of photon I. In other 

words, if the same settings are selected 

for both channels (β = α), the second 

photon is systematically detected in 

the same state as photon I. The measurement 

outcomes on both channels 

I and II are perfectly correlated: that is, 

the conditional probabilities P (V II α |V I α ) 

and P (H II α |H I α ) are equal to 1 whatever 

α is. 

So P (|V I α , |H II α ) = P (|V I α |V II α ) = 1/2 

and P (|H I α , |V II α ) = P (|V I α |H II α ) = 0 

and the degree of correlation of measurements 

between both channels is in this 

case : E = (α, α) = P (|V I α , |V II α ) + P (|H I α , 

|H II α ) – P (|H I α , |V II α ) – P (|V I α , |H II α ) = 1 

Bell’s parameter 

More generally, we can choose to rotate 

the measurement basis on path I 

with an angle α and on path II by an 

angle β, different from α. Quantum 

formalism then predicts that the 

conditional probability of detection 

is expressed as: 

P (|V I α | |V I I β ) = cos 2 (α – β) 

which is maximal and equal to 1, as 

we already noticed, as long as α = β. 

The degree of correlation of the 

measurements between both channels 

is in this case : 

E (α , β ) = P (|V I α , |V I I β ) + P (|H I α ,|H I I β ) 

– P (|H I α ,|V I I β ) – P (|V I α ,|H I I β ) 

E (α , β ) = cos 2 (α , β ) – sin 2 (α , β ) = 

1 –2 cos 2 (α , β ) 

So, with this experiment setup, we 

can measure the Bell’s parameter 

which is : 

S Bell = E (α , β ) + E (α', β ) 

+ E (α', β') + E (α , β') 

The maximal value is obtained for 

α = 22.5°, α' = 45°, β = 45° and β' = 67.5° for 

which the Bell’s parameter is S Bell = 2√ – 2. 

Non-locality in 

quantum correlations 

We must however stress that the formalism 

of quantum mechanics has several 

counterintuitive (maybe 


LABWORK 

even disturbing!) consequences. 

Let us recall that the measurement 

outcomes on photon I are 

uniformly distributed over states 

|V I α and |H I α , and that this property 

does not even depend on α. 

So, when considering the outcome 

of the first measurement occurring 

in the setup, one cannot 

assign any preferred direction to 

any photon whatsoever. The polarizer 

settings on both channels 

can be changed independently 

until the very last moment. Then, 

as soon as projection occurred 

on photon I, quantum formalism 

tells us that the state of photon II 

is also instantaneously projected 

and known with certainty. There 

is no need to explicitly perform 

a measurement on photon II: it 

Figure 1. Classical polarization measurement setup: 

Each channel is composed of a polarizing beam 

splitter (PBS) preceded by a half-wave plate (HWP) 

which allows one to choose the projection basis of 

the polarization measurements (α and β). 

becomes assigned to a specific 

state, not defined by a measurement 

of one of its own features, but 

via a measurement performed on 

another particle, possibly located at 

the opposite side of the universe! This 

instantaneous “spooky action at a 

distance” disrupts our understanding 

of “locality”. 

Figure 2. Experimental setup built in the LEnsE 

in 2005 

EPR paradox: 

The completeness 

of quantum mechanics 

in question 

Entangled states display correlations 

that seem to involve non-local 

influence between physically separated, 

non-interacting systems. This 

deeply troubled many physicists 

including Einstein, and led to the 

famous Einstein-Podolsky-Rosen 

(EPR) paper published in 1935 [4]. In 

this article, the authors imagined a 

similar situation as the one exemplified 

above in a famous thought experiment 

(“Gedankenexperiment”). 

But they also explicitly assumed that 

such spooky action at a distance was 

impossible, so that the quantum 


LABWORK 

It seems that Niels Bohr was deeply troubled 

by the EPR argument relying precisely on quantum 

formalism itself to show its incompleteness. 

He was convinced that if the “EPR” reasoning 

on reality and locality was correct, all of quantum 

physics would collapse. 

physics description failed at giving an appropriate 

understanding of the process. 

Einstein and his partners however believed 

that another theory, compatible 

with local reality (so called hidden variable 

theory or HVT), could explain 

the correlations of entangled states as 

predicted by quantum mechanics. This 

theory was yet to establish. The general 

idea behind local hidden variable 

theories is the following. It is assumed 

that the photon pairs have a new kind of 

physical property: for instance here, an 

identical "polarization property", shared 

by both photons and attributed to them 

via the pair-generation process, and 

labeled, say, by θ. Since θ is the same 

for both photons, it would explain why 

both photons of any pair are measured 

along the same direction whatever this 

direction is. And since it is established 

at the source, it is local: the photons carry 

θ with them at all time. The result of the 

measurement performed on each channel 

depends on the value of θ for the 

photon itself, and not on what happens 

to the other photon. θ is called a hidden 

variable, in the sense that it does not appear 

explicitly in the expression of the 

state, |ψ 2ph , in the quantum formalism. 

This tends to show that the wavefunction 

somehow “lacks” some of the information 

on the system. This is why it is 

said that the EPR paper and the hidden 

variable theories contradict the completeness 

of the quantum theory. 

It seems that Niels Bohr was deeply 

troubled by the EPR argument relying 

precisely on quantum formalism itself 

to show its incompleteness. He was 

convinced that if the “EPR” reasoning 

on reality and locality was correct, the 

whole quantum physics theory would 

collapse. Bohr defended the formalism 

of quantum mechanics by asserting that, 

for these entangled quantum states of 

several particles, one could not speak 

of the individual properties of each of 

the particles: thus, there are no hidden 

variables. Entangled particles definitely 

behave as a single object regardless of 

their separation distance. 

The debate between Bohr and Einstein 

lasted for more than twenty years until 

they both died. In fact, Einstein never 

contested the correctness of quantum 

physics predictions, but how quantum 

physics explained these predictions. 

Bell’s theorem 

and parameter 

But in 1965, surprising breakthrough, the 

Irish physicist John Bell showed that this 

debate could be settled experimentally 

[5]. He showed that if we measure the 

Bell’s parameter, S Bell , as defined before, 

assuming any local hidden variable hypothesis, 

its value is less than two. 

–2 < S Bell < 2 

These are the so-called Bell’s inequalities. 

In parallel, quantum physics predicts value 

of S Bell > 2 for specific choices of α, α', 

β and β' , with a maximal value of S Bell = 

2√ – 2. Entangled states are said to violate 

Bell’s inequalities. 

Since the two formalisms are not compatible, 

then who is right? Einstein or Bohr? 

An experimental test of Bell’s inequalities 

is thus to choose a conflictual set of 

measurements and to measure what is 

the value of Bell’s parameter, and see if it 

is compatible or not with a local hidden 

variable theory. 


LABWORK 

An overview of the 

labwork setup 

This experimental setup was proposed 

in the early 2000’s [2,5]. What 

limited many experiments a few decades 

ago was the difficulty to create 

a bright source of photon pairs. 

A convenient way to proceed is to 

use a type I phase matching spontaneous 

parametric downconversion 

process in nonlinear crystals, see 

figures 2 and 3. The pump, a 60mW 

at 405nm blue InGaN laser diode, gives 

810nm entangled photons. We 

will come back later to the detailed 

description of the Bell’s state preparation. 

Downconverted photons are 

collected by two lenses (focal length: 

75mm, diameter: 12.5mm) at about 

one meter from the crystals and 

focused on single photon counting 

avalanche photodiodes. Filters at 

810nm, 10nm width, are placed just 

in front of each lens. Polarization is 

analysed by rotating the half wave 

plates in front of the polarization 

beam splitter cubes. 

Black plastic tubes prevent from 

stray light and protect single photon 

counting modules. For each detected 

single photon, these modules 

give a 25ns TTL pulse which is sent 

to a coincidence detector to ensure 

that the coincidences are measured 

between photons of the same downconverted 

pair. In our experiment, for 

Figure 3. Two-crystal downconversion source: 

The crystals are 0.5 mm thick and in contact 

face-to-face, while the pump beam is approximately 

1 mm in diameter. 

single detection rates of about 

23000 on each side, we detect a coincidence 

rate of about 1600 (that is 

1600 pairs of photons persecond). 

Bell state 

preparation 

The key of the setup is the preparation 

of a pure entangled state, one 

that will make sure that we can discriminate 

between hidden variable 

theories and quantum mechanics. 

We shall create photon pairs 

through a process that will indistinguishably 

generate either |H I |H I I 

or |V I |V I I in a superposition state 

(and not in a statistical mixture). 

To produce polarization entangled 

pairs of photons, we use two 

identical thin crystals, rotated by 

90° from each other about the pump 

beam direction (see figure 3). For 

a vertically polarized pump, the 

down-conversion process generates 

pairs of horizontally polarized 

photons in the first crystal; the second 

crystal has no more effect. 

For a horizontally polarized pump, 

the first crystal has no effect; the 

down-conversion process generates 

pairs of vertically polarized photons 

in the second crystal. 

|V pump → e iH |H I |H II 

and |H pump → e iV |V I |V II 

For a rectilinearly polarised pump 

at 45°, the pump photons state is 

written as: 

|ψ pump = 1 — √ 

– 2 

(|V pump + |H pump) 

In this configuration, 45° polarized 

pump photons can down-convert in 

either crystal. But it is absolutely 

impossible to know in which crystal 

the photon pairs were created! 

By erasing this “which crystal information”, 

we ensure that the photon 

pairs are in the superposition state: 

|ψ 2ph = 1 — √ 

– 2 

(|V I |V II + e i 0 |HI |H II ) 

Where 0 is a phase which depends 

on many different parameters 

(wavelengths of pump and downconverted 

photons, for example), 

and is a direct consequence of the 

birefringence of the crystals. 

In practice, to get a pure Bell’s state, 

we pre-compensate this phase by 

placing a Babinet compensator in 

front of the pair of crystals. The role 

of the Babinet compensator is to introduce 

a relative phase between 

|H pump and |V pump: 

Babinet 

|ψ Pump = — 1 – (|V pump – e -i 0 |Hpump ), 

√ 2 


LABWORK 

The measurement of a Bell parameter is now 

routinely used with various quantum systems, 

in order to evaluate their performances in the context 

of quantum technologies. 

leading to a pure Bell’s state : 

|ψ EPR = 1 — √ 

– 2 

(|V I |V II + |H I |H II ), 

For this state, the joint probability of 

detection in the V I 

45° 

|V II 

45° 

| configuration 

reaches a maximum. The Babinet 

compensator is adjusted while monitoring 

the coincidence rate until it 

reaches an optimum for α = β = 45°. 

Results 

The Bell parameter is measured by 

using a configuration of the four measurement 

basis that maximizes the deviation 

between local hidden variable 

theories and quantum mechanics. It is 

given by the angles α = 0°, β = 22.5°, 

α' = 45°, β' = 45°. 

In our experiment, we obtain S bell (α, α', 

β, β') = 2.48 with a standard deviation 

σ = 3.10 -3 . The result shows that we do 

not reach an ideal entanglement quality 

(S bell = 2√ – 2), but it clearly and unambiguously 

disagrees with any hidden 

variable local theory. 

Conclusion 

Since the eighties, the EPR paradox is no 

longer a “Gedankenexperiment”. Now, 

it has become a very exciting labwork 

for students. It also has recently taken 

another dimension: the measurement 

of a Bell parameter is now routinely 

used with various quantum systems, in 

order to evaluate their performances in 

the context of quantum technologies. 

This labwork therefore now echoes 

with other ambitions aiming at building 

more advanced experimental training 

platforms for the second quantum revolution. 

Talking about indistinguishability, 

the Lense has been proposing for 

now more than 8 years a labwork dedicated 

to the Hong-Ou-Mandel effect, a 

two-particle interference experiment 

allowing to quantify the indistinguishability 

of photons generated by 

a similar downconversion process. 

Another setup under construction will 

be dedicated to the study of nitrogen 

vacancy centres in diamond, and their 

behaviour as single photon sources. 

REFERENCES 

[1] A. Aspect, “Bell’s Theorem: The Naive View of an Experimentalist,” Quantum [Un] 

speakables, 119–153 (2002). https://doi.org/10.1007/978-3-662-05032-3_9. 

[2] D. Dehlinger, M. W. Mitchell, Am. J. Phys. 70, 903–910 (2002). https://doi. 

org/10.1119/1.1498860. 

[3] C. Fabre, Photoniques 107, 55-58 (2021) https://doi.org/10.1051/photon/202010755 

[4] A. Einstein, B. Podolsky, N. Rosen, Phys. Rev. 47, 777–780 (1935). https://doi. 

org/10.1103/PhysRev.47.777. 

[5] J. S. Bell, Phys. Phys. Fiz. 1, 195–200 (1964). https://doi.org/10.1103/ 

PhysicsPhysiqueFizika.1.195. 

[6] D. Dehlinger, M. Mitchell, Am. J. Phys. 70, 898–902 (2002). 

https://doi.org/10.1119/1.1498859. 



SURFACE plasmon imaging 

SURFACE PLASMON 

PROPAGATION IMAGING 

BY A PHOTON SCANNING 

TUNNELING MICROSCOPE 

/////////////////////////////////////////////////////////////////////////////////////////////////// 

F. DE FORNEL 1, *, P. DAWSON 2 , L. SALOMON 1 , B. CLUZEL 1 

1 

Laboratoire Interdisciplinaire Carnot de Bourgogne, Université de Bourgogne, 9 Avenue Alain Savary, 21078 Dijon, France 

2 

Centre for Nanostructured Media, School of Mathematics and Physics, Queen's University, Belfast, BT7 1NN, UK 

*ffornel@u-bourgogne.fr 

This paper describes the first direct observation 

of surface plasmon propagation on a metallic thin 

film deposited on a glass slide, thus facilitating the 

first direct measurement of the propagation length. 

This was achieved using photon scanning tunneling 

microscopy (PSTM). Subsequently, near-field 

observations of the surface plasmon buried at the 

metal-glass interface, generated from a discontinuity 

in the metallic thin film, were reported. 


In the 1990’s, near-field optical 

microscopies took off promising 

to become a unique tool 

for exploring optical phenomena 

at the nanoscale. 

At that time, the photon 

scanning tunneling microscope 

(PSTM) [1,2] which was the analog 

for photons of the electronic scanning 

tunneling microscope (STM) 

demonstrated these capabilities. 

Using this technique, it became 

possible to measure the optical near 

field in the vicinity of subwavelength 

structures illuminated under 

various conditions. Among them, 

the PSTM has proven to be a powerful 

tool for the study and the analysis 

of surface plasmons which are 

resonant electromagnetic modes 

associated with the collective oscillation 

of an electron plasma at 

the interface between a metal and 

a dielectric [3]. Indeed, for a thin 

film of silver deposited on glass, the 

metal-air surface plasmon excited 

in Kretschmann configuration [4] 

has an extension in air of a few 

hundred nanometers in the visible 

range, making it an ideal candidate 

for optical near-field measurement 

with a PSTM. In the seminal paper 

of Dawson et al [5], the propagation 

of a surface plasmon was shown for 

the first time and its damping was 

directly quantified by viewing its 

propagation length. These results 

were further completed by the demonstration 

of the contribution of 




a buried metal-glass plasmon in the 

near- field pictures thanks to successive 

improvement in near-field measurement 

techniques. 

DIRECT OBSERVATION OF THE 

METAL-AIR SURFACE PLASMON 

Figure 1 shows the schematic of the 

seminal experiment performed at 

the University of Bourgogne. A 50 nm 

thin silver film was illuminated by two 

Helium:Neon lasers in oblique incidence. 

A first, green one at λ 1 =543,5nm 

was unfocussed to yield a spot size on 

mm 2 scale on the silver film while the 

second one at λ 2 = 632.8 nm was tightly 

focused into a ~10 µm diameter spot at 

the silver film. The angle of incidence 

of the red beam was chosen so that it 

excited the silver-air surface plasmon 

by phase matching the parallel (to the 

sample interface) component of wavevector 

of the incident light to that of the 

surface plasmon. In plasmonics this 

phase matching technique is well known 

as the Kretschmann configuration [4]. 

The angle of incidence of the green laser 

was likewise chosen to excite 

Figure 1. Schematic of the experiment: a silver film evaporated on a glass prism was 

illuminated by two lasers. The green one acts as a pilot for scanning a tapered optical fiber 

in the near-field of the silver film while the red one excites the silver-air surface plasmon 

in Kretschmann configuration. 




a) 

b) 

surface plasmons at the silver-air interface 

at that wavelength. An optical 

probe, obtained by tapering an optical 

fiber, was brought into the optical 

near field of the metal film in order 

to ‘collect the surface waves’ (i.e. by 

scattering the surface-bound optical 

field of the surface plasmons to photons 

travelling up the tapered optical 

fiber) while scanning the metal film 

with help of a home-made stack of 

piezoelectric tubes. 

In this configuration, the nearfield 

probe simultaneously detects 

signals coming from the two lasers 

at 632.8 nm and 543.5 nm. A fiber optic 

coupler then separates the signal 

in two arms. On one arm, a spectral 

Figure 2. a) PSTM image of the optical near field intensity 

of the incident beam, λ 1 = 543.5 nm, on a simple prism 

illuminated under total internal reflection. b) PSTM 

image recorded at λ 2 = 632.8 nm at the surface plasmon 

excitation angle for red light on a section of the same 

prism covered by a 50nm thin silver film. The right side 

of the image displays a clear exponentially decaying tail 

explicitly demonstrating the propagation and damping 

of the surface plasmon. All images are 40 µm x 40 µm. 

Figure 3. a) Scheme of the experiment. A semiinfinite 

thin gold film of thickness h=55 nm is vacuum 

deposited onto a glass prism. The illumination is carried 

out at various incident angles θ, by a collimated laser 

beam preliminary linearly polarized. The optical signal 

is collected via the probe and a photomultiplier tube 

(PMT). b) PSTM image operating in a constant intensity 

mode for an angle of incidence of 52° 

filter selected the green laser which 

acts as a pilot for driving a feed-back 

loop allowing the probe to scan the 

surface without touching it. For this 

reason, the green laser spot is large 

(mm scale) in order to generate a 

surface intensity distribution on 

the topside of the silver film that is 

(near-) constant on the ~10µm scale 

of the surface plasmon propagation 

length. The green signal is used as 

the input signal for the feedback 

loop of the piezoelectric scanners 

to allow for scanning the silver film 

at a constant height. On the second 

arm, the red laser exciting the surface 

plasmon is filtered out and then 

sent to a photomultiplier while the 

a) 

b) 




This was the first direct observation of the 

propagation of a surface plasmon on top of a metal 

film by near- field imaging paving the way to many 

other achievements and realizations in this field and 

development of commercial near-field microscopes 

for studying the optical near-field at nanoscale. 

probe is scanning the surface. At 632.8 

nm, the typical images reported in [5] 

are displayed in Fig. 2. It shows the 

original near-field pictures obtained 

in the same conditions on a section 

of the glass slide without the silver 

film (Fig.2a) and with the silver film 

(Fig.2b). Without the silver film, the 

measured laser spot is approximately 

circular with a ~ 10 µm diameter, while 

with the silver film the right side of the 

imaged spot appears elongated with an 

SURFACE PLASMON POLARITON 

exponential decay directly mapping 

the surface plasmon excitation, propagation 

and attenuation. 

From the image of Fig. 2b) the 

propagation length was measured 

to be 13.6µm, which is less than that 

predicted for a thin silver film of this 

thickness. The difference was attributed 

to re-radiation of the surface 

plasmon back into the prism as it propagates, 

contamination of the silver 

surface with a thin, optically 

A surface plasmon polariton (SPP) is an electromagnetic excitation 

that propagates in a wave like fashion along the planar interface 

between a metal and a dielectric medium. Thus, a SPP is a surface 

electromagnetic wave, whose electromagnetic field is confined to 

the near vicinity and whose amplitude decays exponentially with 

increasing distance into each medium from the interface of the 

dielectric–metal interface. For a film deposited on glass, plasmons 

at each interface can exist. 

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dissipative silver sulphide layer and 

a possible angular shift between 

the theoretical and actual angle of 

incidence. This was the first direct 

observation of the propagation of a 

surface plasmon on top of a metal 

film by near- field imaging paving the 

way to many other achievements and 

realizations in this field and the development 

of commercial near-field 

microscopes for studying the optical 

near-field at nanoscale. 

However a metallic thin film has 

two interfaces and further studies 

were then conducted to explore the 

surface plasmon buried at the metal-glass 

interface, which is not excited 

in Kretschmann configuration. 

BURIED SURFACE 

PLASMON OBSERVATION 

In this second experiment, a semi-infinite 

metallic film was deposited 

on glass prism as shown in 

Fig. 3. It is noteworthy that here, 

gold was preferred to silver to avoid 

sulfurization. Under collimated illumination, 

the edge of the metal 

film scatters the incident laser and 

generates both the metal-air and 

the metal-glass surface plasmons. 

First measurements in this configuration 

were reported in [6] which was 

employing the same laser for exciting 

surface plasmons as well as for driving 

the piezoelectronic scanners through 

a feedback loop. In this configuration, 

the height of the probe during the scan 

was modulated by the detected light 

intensity. Recorded near-field images 

showed both an exponential decay related 

to the propagation and damping 

of the gold:air surface plasmon and 

but also the presence of oscillations 

whose period corresponds to an interference 

between the incident light and 

the gold:air plasmon. These results 

were in very good agreement with numerical 

simulations. However, these 

latter also predicted that interferences 

between the glass-to-metal plasmon 

and the incident beam should also be 

visible in the measurements but was 

not observed experimentally. This was 

finally achieved by Brissinger et al. [7] 

Figure 4. IInterference periods Λ i in near-field 

images obtained in the experiment described in 

Fig.3a by varying the angle of incidence. Λ 1 and Λ 2 

are associated with the interferences of the incident 

wave and the plasmonic waves, respectively at the 

m/a and the m/g interfaces. Λ 3 is associated with 

the interferences between both surface plasmon 

waves and Λ 4 between the incident wave and the 

back reflection at the prism/air interface. The solid 

and dotted lines are the values from the numerical 

simulations and the crosses are recovered from the 

near-field optical images. 

If the photon scanning 

tunneling microscope in 

constant optical intensity 

mode is no longer used, it 

has nevertheless allowed the 

direct and first imaging of 

the surface plasmon excited 

at oblique incidence in 

Kretschmann configuration. 

REFERENCES 

[1] R.C. Reddic, R.J. Warmack, T.Ferrel, Phys. Rev. B 39,767-770 (1989) 

[2] D. Courjon., K. Saraeyeddine ., M. Spajer, Opt. Commun. 71, 23 (1989) 

[3] A. Zayats, I. Smolyaninov , A. Maradudin, Phys. Rep. 408, 131 – 314 (2005) 

[4] E. Krestschman, H. Rather, Z. Naturforsch, A23, 2135 (1968) 

[5] P. Dawson et al, Phys. Rev. Lett. 72, 2927 (1994) 

[6] L. Salomon et al., Phys. Rev B 65, 125409 (2002) 

[7] D. Brissinger et al, Opt. Express 19, 17750 (2011) 

many years after, benefiting from 

a shear-force feedback for driving 

the near-field probe with an improved 

reliability. 

At that point, the angle of incidence 

was tuned from 30° to 60° and both surface 

plasmons were finally visible in 

the near-field images. Interferences 

were in very good agreement with 

numerical predictions. It should be 

noted that the wavector of the buried 

surface plasmon, that of the metal: 

glass interface, was recovered experimentally 

(1.63±0.08) ω/c quantitatively 

compared to its theoretical value 

of 1.59ω/c. 

CONCLUSION 

If the photon scanning tunneling 

microscope in constant optical intensity 

mode is no longer used, it 

has nevertheless allowed the direct 

and first imaging of the surface plasmon 

excited at oblique incidence in 

Kretschmann configuration As such 

these experiments have paved the way 

to the emergence of plasmonics as 

an entire field of research which has 

enabled applications in optoelectronics, 

medicine, chemistry, or sensing 

to cite just a few. In addition, current 

improvements in the stability of feedback 

loops, piezoelectric scanners, 

nanotechnologies used for shaping 

near-field probes, as well as recent 

developments in laser technologies 

have allowed the near-field microscope 

to move beyond the walls of the 

laboratory and to become a standard 

imaging tool for materials science. 


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comb lines narrows down to below 1 

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of 10 -15 or better. The newest generation 

of optical atomic clocks enabled 

by this technology reach an accuracy of 

10 -18 [3], two orders of magnitude higher 

than the best cesium atomic clock. 

Menlo Systems' FC1500-Quantum is 

a complete CW laser system with an ultra-stable 

frequency comb. It provides 

several CW lasers for atom cooling, repumping, 

and addressing the sub-Hertz 

linewidth clock transition in atoms or 

ions used in optical clocks (see figure). 

The low phase noise obtained on the 

comb-disciplined CW lasers is essential 

for coherent gate manipulation in many 

atom optical quantum computing schemes 

and for fast and high-fidelity gate 

operations. The laser light is delivered 

via optical fiber to the "physics package" 

consisting of vacuum chamber with all 

the necessary optics and electronics 

components and the atoms. Labs no longer 

need to undergo the time consuming 

Figure: The hyperfine 

transitions in strontium (Sr) 

atoms with the ultra-narrow 

clock transition at 698.4 nm 

(upper part). A commercial 

FC1500-Quantum system for 

optical clock applications 

contains an ultra-stable laser 

transferring its spectral purity 

onto an optical frequency 

comb and all other lasers 

which are also locked to the 

comb (lower part). 

process of designing and building their 

own ultra-stable lasers. Eventually, the 

comb itself has two purposes: It acts as 

reference for all the lasers to maintain 

their narrow linewidth, and it is the 

clockwork that transfers the optical 

frequency’s spectral purity to the microwave 

region, or to a different optical 

frequency [4]. 

CONTACT: 

Menlo Systems GmbH 

Bunsenstraße 5 

82152 Martinsried, Germany 

Phone: +49 89 189166 0 

Fax: +49 89 189166 111 

sales@menlosystems.com 

www.menlosystems.com 

REFERENCES 

[1] J. Reichert et al., Opt. Commun. 172, 

59 (1999) 

[2] R. W. P. Drever et al., Appl. Phys. B. 31, 

97 (1983) 

[3] E. Oelker et al., Nat. Photonics 13, 

714 (2019) 

[4] M. Giunta et al., Nat. Photonics 14, 

44 (2020) 


FOCUS 

OPTICAL FREQUENCY combs 

INTERFEROMETRY 

WITH OPTICAL FREQUENCY COMBS 

/////////////////////////////////////////////////////////////////////////////////////////////////// 

Nathalie PICQUÉ 1,* , Theodor W. HÄNSCH 1,2 

1 

Max-Planck Institute of Quantum Optics, Garching, Germany 

2 

Ludwig-Maximilian University of Munich, Faculty of Physics, Munich, Germany 

*nathalie.picque@mpq.mpg.de 


A 

frequency comb [1] 

is a spectrum of narrow 

evenly spaced 

laser lines, whose 

absolute frequency 

can be known within 

the accuracy of an atomic clock 

(Insert 1). Initially invented for precision 

frequency metrology in the 

simple hydrogen atom, frequency 

combs have become key to a variety 

of applications, from the generation 

of attosecond pulses to the calibration 

of astronomical spectrographs. 

They are enabling ground-breaking 

approaches to interferometry with applications 

as diverse as spectroscopy 

[2], distance metrology [3], or holography 

[4]. This short article recounts the 

principle and applications of frequency 

comb interferometry. 


AND INTERFEROMETRY 

Early on, frequency combs have been 

coupled to known interferometers. 

A frequency comb, a spectrum of equidistant 

phase-coherent laser lines, can be harnessed for 

new approaches to interferometry. The dual-comb 

interferometer exploits the time-domain interference 

between two combs of slightly different line spacing. 

The instrument, which performs direct frequency 

measurements over a broad spectral bandwidth, 

opens up new perspectives in applications such as 

spectroscopy, distance metrology or holography. 

For example, if a frequency comb 

is used as a light source before a 

scanning Michelson interferometer, 

Fourier transform spectroscopy 

sees its measurement speed, 

sensitivity precision and accuracy 

dramatically improved. Distance 

measurements using a spectrally-dispersed 

static Michelson interferometer 

-or a scanning Michelson 

interferometer- benefits from the 

many comb lines for improved precision 

and large ambiguity range. 

A comb of narrow line spacing can 

be frequency filtered in a Fabry 

interferometer, of matching but 

larger free spectral range, to generate 

a comb of large repetition frequency, 

suited to the calibration of 

astronomical spectrographs. The 

Fabry-Pérot resonator can also be 

used as an enhancement cavity, for 

extreme-ultraviolet high-harmonic 

generation or for absorption 

spectroscopy with long absorption 

paths. 

DUAL-COMB INTERFEROMETERS 

More interestingly, frequency 

combs have enabled a new class of 

interferometers, called dual-comb 

interferometers. Two frequency 

comb generators emit trains of 

pulses at slightly different repetition 

frequencies, f rep and f rep +Δ f rep , 

respectively. The two beams of the 

two combs are combined on a beam 

splitter and their interference is 

measured on a fast photodetector as 

a function of time. In the time domain 

(Fig.1a), the pulses from one 

laser walk through the pulses from 

the second laser, with a time separation 

that automatically increments 

from pulse pair to pulse pair by an 

amount Δ f rep / f rep 

2 

(e.g. 10 - 14 s). This 

way, optical delays from 0 to 1/f rep 

are repetitively scanned without 

moving parts. Akin to a sampling 

oscilloscope, the periodic optical 

waveforms are stretched in time 

by a factor f rep /Δf rep (e.g. 10 6 ), and 

they can be electronically recorded 



FOCUS 

and digitally processed. In the frequency 

domain (Fig.1b), pairs of 

optical comb lines, one from each 

comb, produce radio-frequency 

beat notes on the detector, forming 

a frequency comb of line spacing 

Δf rep in the radio frequency domain. 

Optical frequencies nf rep +f 0 are thus 

down-converted into radio frequencies 

nΔf rep +Δf 0 . 

The principle of dual-comb 

interferometry may be reminiscent 

of that of asynchronous 

optical sampling. A significant 

difference, though, is that mutual 

coherence between the two combs 

is required for an interferometric 

measurement. As a rule of thumb, 

many applications require a relative 

stability on the order of λ/100 

to achieve high fringe contrast. At a 

wavelength of 1 µm, this means that 

the timing jitter between pairs of 

pulses must be kept smaller than 30 

as. Learning how to control and minimize 

the relative timing and phase 

fluctuations between two frequency 

comb generators has been at the center 

of significant efforts in the early 

days of dual-comb interferometry. 

A number of solutions, of various 

complexity and performance, now 

enable to experimentally maintain 

mutual coherence and/or to compensate 

for the (residual) fluctuations 

through analog or digital processing. 

Experimentally, coherent averaging 

over more than half an hour 

has become feasible, illustrating the 

degree of control achieved for a dualcomb 

system. 

Figure 1. a. Time-domain principle 

of dual-comb interferometry (in 

the situation where an absorbing 

sample is in the beam path of 

comb 1). b. Frequency-domain 

principle c. Sketch of a typical dualcomb 

spectrometer. c.: Adapted 

from "Mid-IR Spectroscopic 

Sensing," Optics & Photonics News 

30(6), 26-33 (2019). https://doi. 

org/10.1364/OPN.30.6.000026 


FOCUS 


FREQUENCY COMB 

1/f rep 

φ 

A frequency comb, a spectrum of 

equidistant phase-coherent spectral 

lines, is commonly generated by 

a mode-locked laser. Such a laser 

emits a periodic train of pulses at a 

frequency f rep . The periodicity applies 

not only to the pulse envelopes, but to 

the whole electric field of the pulses, 

including their optical phase, apart 

from a reproducible slip φ of the 

phase of the electromagnetic carrierwave 

relative to the pulse envelope 

from pulse to pulse. Such phase slips 

occur in a laser owing to dispersion 

in the cavity. As a consequence, the 

frequency f n of a comb line writes: 

f n = n f rep + f 0 where n is an integer and 

f 0 = f rep φ/ 2π is the carrier-envelope 

offset frequency. Both f rep and f 0 can 

be measured and controlled against 

an atomic clock. Therefore, frequency 

combs act like rulers in frequency 

space that can for instance be used to 

measure a large separation between 

two different optical frequencies in 

terms of the countable signal of the 

pulse repetition frequency. Frequency 

combs can conveniently link optical 

and microwave frequencies and 

enable absolute measurements of 

any frequency. Frequency combs have 

revolutionized the way the frequency 

of light is measured and are now 

common equipment in all frequency 

metrology-oriented laboratories. 

They have paved the way for the 

creation of all-optical clocks with 

a precision that approaches the 

10 -18 level. 

2φ 

f rep 

nf rep +f 0 

time 

frequency 

Owing to the absence of moving 

parts, one of the first features of dualcomb 

interferometers that attracted 

interest has been the ability to perform 

fast measurements. Moreover, the sensitivity 

is further enhanced. is further 

enhanced by the use of coherent light 

sources. Remarkably, a dual-comb 

interferometer can simulate a mirror 

moving at 10 km.s -1 , the escape velocity 

from earth. The beat notes between 

pairs of comb lines are mapped in 

the radio-frequency domain where 

the 1/f noise in the detector signal can 

be greatly reduced, and a million-fold 

improvement in the acquisition speed 

of a spectrum has been demonstrated. 

Furthermore, all the spectral elements 

are simultaneously measured on a 

single photodetector, like in other interferential 

spectrometers. This multiplexed 

recording grants excellent 

overall consistency of the spectral 

measurements and applicability in 

any spectral regions. As research progressed, 

other distinguishing assets 

have been highlighted, offering interferometry 

a unique and novel host of 

powerful features, which are summarized 

below for the key applications. 

DUAL-COMB SPECTROSCOPY 

The main application of dual-comb 

interferometers has been spectroscopy 

over broad spectral bandwidths, in 

particular of molecules [2]. Dual-comb 

spectroscopy is a new form of Fourier 

transform spectroscopy, which has 

been over the past 50 years the overriding 

tool in molecular spectroscopy 

and analytical chemistry. If a sample is 

present on the beam path of one of the 

combs (Fig.1c), the interference signal 

will record the signature of the absorption 

and dispersion experienced by 

the sample. The dual-comb spectrometer 

mimics a scanning Michelson 

interferometer by sampling the freeinduction 

of the molecules over the 

range of optical delays. One uses then 

a harmonic-analysis tool, the Fourier 

transformation, to get the spectrum. 

In a single recording, the spectrum is 

sampled by the discrete comb lines 

and therefore the spectral resolution 

is limited to the comb line spacing 

f rep . As most combs can be precisely 

controlled, it is however possible to 

measure a sequence of spectra with 

different comb-line positions and to 

interleave the spectra a posteriori to 

reach a resolution ultimately limited 

by the width of the optical comb lines. 

Initially developed in the near-infrared 

spectral region, where frequency 

comb generators are conveniently 

available, dual-comb spectroscopy is 

now efficient in the mid-infrared spectral 

region at wavelengths as long as 

5 µm (Fig. 2). It is emerging at longer 

wavelengths in the mid-infrared and 

THz ranges and at shorter wavelengths 

in the visible range. The ultraviolet domain 

is still mostly unexplored, due to 

outstanding instrumental challenges 

for generating low-noise frequency 

combs of a broad span and implementing 

ultra-stable interferometers 

at short-wavelengths. 

To broadband spectroscopy, dualcomb 

interferometers add the remarkable 

features of the interrogation 

of the sample by laser lines of narrow 

width which provides a negligible 

contribution of the instrumental lineshape, 

as well as the calibration of the 

frequency scale within the accuracy 

of an atomic clock. These features 

are not available from any dual-comb 

interferometers though. They derive 

from the use of frequency combs of 

narrow and stable optical lines, referenced 

to a radio-frequency clock or to 

an ultra-stable optical frequency standard. 

Initially, it was not clear which 

light sources should be used and a variety 

of comb generators have been 

tested, from the metrology-grade 

fiber mode-locked lasers stabilized to 

accurate optical references to simple 

electro-optic modulators or quantum 

cascade lasers that are free-running. 

Over the years, the fiber-laser technology 

has considerably evolved towards 

ease of use, compactness, low noise 

and ultra-high stability. Fiber laser 

have become the most suited tools for 

realizing the potential of dual-comb interferometry. 

Free-running frequency 

combs do not enable to benefit from 



FOCUS 

the above-mentioned advantages, 

although they might still be valuable 

tools in some niche applications. 

Although linear absorption spectroscopy 

has been predominantly 

explored, frequency combs synthesizers 

based on mode-locked lasers 

involve intense ultrashort pulses that 

can generate nonlinear phenomena at 

the sample. Using this, various schemes 

of nonlinear Raman spectroscopy 

and imaging and of Doppler-free 

two-photon excitation spectroscopy 

[5] have open up new opportunities 

for nonlinear spectroscopy over broad 

spans. One can expect that this line of 

research will develop further in the 

near future, owing to the progress in 

high-power laser amplifiers at high 

repetition frequency. Finally, the system 

is not limited to two frequency 

combs and recent proof-of-principle 

demonstrations explore the intriguing 

potential of using three combs 

for multidimensional spectroscopy, 

such as photon echoes [6]. 

DUAL-COMB RANGING 

AND HOLOGRAPHY 

Another successful application of 

dual-comb interferometers involves 

distance measurements and 

ranging [3]. The dual-comb scheme 

combines the time-of-flight and interferometric 

approaches to deliver 

absolute distance measurements 

over an extended ambiguity range. 

One of the most exciting recent 

Figure 2. Mid-infrared 

experimental dualcomb 

spectrum, 

shown with different 

magnifications. (a.-c.) 

82000 comb lines 

spaced by 100 MHz, 

centered at 92 THz 

(3.2 µm), were 

measured within 

29 minutes. In b. 

an absorption 

transition of ethylene 

attenuates the comb 

lines. Reproduced 

from “Mid-infrared 

feed-forward dualcomb 

spectroscopy,” 

Proc. Natl. Acad. 

Sci. USA 116, 34549 

(2019). https:// 

doi.org/10.1073/ 

pnas.1819082116 

trends, though, has been to replace 

the single photodetector by a camera 

sensor. As many spectra as there are 

detector pixels can be simultaneously 

measured. Dual-comb interferometers 

move digital holography 


FOCUS 


forward by recording simultaneously 

thousands of holograms [4], one per 

comb line (Fig.3), and they show an 

intriguing potential for reaching new 

frontiers in scan-free wavefront reconstruction. 

By digital processing, 

each hologram provides a 3-dimensional 

image of the scene, where the 

focusing distance can be chosen at 

will. Combining all these holograms 

renders the geometrical shape of the 

3-dimensional object with very high 

precision and without ambiguity. 

At the same time, other diagnostics 

can be performed by the frequency 

combs: in the first proof of concept, 

molecule-selective imaging of a 

cloud of ammonia vapor was simultaneously 

demonstrated. 

CONCLUSION 

Dual-comb interferometry has been 

developed over the past 15 years and 

now involves a vibrant research community 

of more than 200 research 

groups. The dual-comb interferometer 

provides a unique combination 

of broad-spectral-bandwidth, long 

temporal coherence, absence of 

moving parts and multi-heterodyne 

read-out which offers interferometry 

a revolutionary host of features – 

frequency multiplexing, resolution, 

accuracy, precision, speed. Other 

Figure 3: In dual-comb holography, as 

many holograms as there are comb lines 

are generated. 

emerging applications include analog-to-digital 

conversion, two-way 

time and frequency transfer, vibrometry, 

etc. 

Excitingly, the technique is still 

far from realizing its full potential. A 

remarkable difference with respect 

to other types of interferometers is 

that the dual-comb interferometer 

performs direct time/frequency 

measurements. With a dispersive 

or interferential instrument, the 

resolving power is the ratio of the 

maximum path difference to the 

wavelength, hence the bulky instruments 

for high resolution. With 

REFERENCES 

a dual-comb interferometer, it becomes 

the ratio of the maximum 

optical retardation to the period of 

the optical wave. Dual-comb spectroscopy 

is therefore the only technique 

that can, for any spans and any 

spacing, potentially reach a resolution 

equal to the comb line spacing, 

freed from geometric limitations 

and aberrations. This apparently 

simple but fundamental distinction, 

first pointed out in [2], has not 

been exploited yet in experiments 

that would go significantly beyond 

the state of the art. On an applied 

touch, the path is open to integrated 

chip-scale ultra-miniaturized devices 

that combine high resolution 

and broad spectral bandwidth. 

Using III-V-on-silicon mode-locked 

lasers in the telecommunication 

region, an on-chip spectroscopy laboratory 

for gas sensing is already 

underway using battery-operated 

devices of a footprint smaller than 

1 mm 2 [7]. The most exciting prospect 

is nevertheless that of merging 

frequency metrology and broadband 

spectroscopy: with Doppler-free 

dual-comb spectroscopy, the envisioned 

improvement in accuracy is 

expected similar to that achieved in 

the 1990s when going from optical 

wavelength metrology to frequency 

measurements. As the technique is 

able to measure very faint signals, 

down to the single-photon level, 

single atoms and molecules may 

become observable over a broad 

span with an unmatched precision, 

opening up new strategies for tests 

of fundamental physics. 

[1] T. Udem, R. Holzwarth, T.W. Hänsch, Nature 416, 233 (2002) 

[2] N. Picqué, T.W. Hänsch, Nat. Photon. 13, 146 (2019) 

[3] I. Coddington, W.C. Swann, L. Nenadovic et al. Nat. Photon. 3, 351 (2009) 

[4] E. Vicentini, Z. Wang, K. Van Gasse et al. Nat. Photon. 15, 890 (2021) 

[5] S.A. Meek, A. Hipke, G. Guelachvili et al. Opt. Lett. 43, 162 (2018) 

[6] J.W. Kim, J. Jeon, T.H. Yoon et al., J. Opt. Soc. Am. B 39, 934 (2022) 

[7] K. Van Gasse, Z. Chen, E. Vicentini, et al. preprint at arXiv:2006.15113 (2020) 



FOCUS 

OPTICAL FREQUENCY 

COMBS FOR 

ATOMIC CLOCKS 

AND CONTINENTAL 

FREQUENCY 

DISSEMINATION 

///////////////////////////////////////////////////////////////// 

Rodolphe LE TARGAT*, Paul-Eric POTTIE, Yann LE COQ 

LNE-SYRTE, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, 

61 Avenue de l’Observatoire, F-75014, Paris, France 

*Rodolphe.LeTargat@obspm.fr 


This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), 

which permits unrestricted use, distribution, and reproduction in any medium, provided 

the original work is properly cited. 

Nearly 25 years ago, Optical 

Frequency Combs (OFCs) 

have revolutionized practically 

from one day to the 

next the metrology of optical 

frequencies. Before that, only 

a few labs in the world could 

connect the optical (~10 15 Hz) 

and the microwave (~10 10 Hz) 

domains, at the price of the 

complex operation of a chain of multiple non-linear frequency 

conversions. In contrast, OFCs provided a tabletop 

instrument, compact, reliable and operable by a single 

scientist. Unmistakeable sign: only a few years had passed 

before pioneering inventors of OFCs Theodor Hänsch and 

John Hall were awarded the 2005 Nobel prize in Physics, 

together with Roy Glauber. 


FOCUS 


The principle of operation 

relies on a femtosecond 

pulsed laser, 

featuring a spectrum 

of typically ~100 000 

phase-locked modes 

contributing simultaneously. The 

field can therefore be defined as: 

iφn (t) 

E comb (t) = ∑E n e 

n 

The average value of each phase 

φ n (t) can be described by 2π(nf rep 

+ f 0 )t + φ n (0), where f rep is the repetition 

rate, f 0 the offset frequency, 

n an integer number and φ n (0) a 

mode-dependent offset . The Fourier 

transform of this field shows that a 

comb is strictly equivalent to a ruler 

in the frequency space: while f 0 

is the offset of the first tooth of the 

ruler, f rep is the difference between 

adjacent teeth. Very importantly, the 

coherence is preserved throughout 

the whole spectrum, which makes 

it a unique tool to connect reliably 

very different spectral domains. 

Beatnotes between an oscillator at 

ν osc and the comb feature a frequency 

at f osc = ν osc - n osc f rep - f 0 , which leads 

to a complete determination of ν osc 

provided parameters f osc , n osc , f rep and 

f 0 are measured accurately. 

The advent of OFCs renewed 

considerably the interest of the Time 

and Frequency community in the 

metrology of optical atomic transitions. 

Even though the definition of 

the SI second had been based on the 

hyperfine transition of Cs since 1967, 

researchers early realized that the use 

of atomic transitions in the optical 

domain would dwarf radically most 

of possible systematic shifts affecting 

clock frequencies. Nevertheless, 

if single ion clocks had been under 

development since the early 80’s, the 

very cumbersome connection to the SI 

second was considerably hindering the 

perspective of a new definition. OFCs 

changed completely this perspective, 

and it is about at the same time, in 2003, 

that a researcher from the University 

of Tokyo, Hidetoshi Katori, proposed 

a scheme allowing the trapping and 

high resolution spectroscopy of many 

neutral atoms simultaneously. The race 

between optical clocks based either 

on single ions or neutral atoms had 

started and is still ongoing today: with 

published uncertainties down to 1×10 -18 

or below, instruments operated notably 

with Sr, Yb, Hg, Yb + , Al + or Sr + 

have outperformed even state-of-theart 

Cs clocks by more than 2 orders of 

magnitude. OFCs are used to compare 

optical clocks either to the SI second 

or directly one with another: the span 

of the comb spectrum is so large that 

it allows comparing clocks that are 

hundreds of nanometers apart. In the 

perspective of the roadmap for a new 

definition the second, aiming at 2026, 

possible contenders to new primary 

and secondary representations of the 

SI second are thus compared frequently 

and reliably in order to progressively 

refine, year after year, the knowledge 

of their frequency ratios. 

Progressively, the technology supporting 

OFCs improved and industrial 

devices became available. If the 

original mode-locked sources were 

Titanium-Sapphire lasers, they often 

missed the reliability necessary to 

measurements bound to last days, if 

not months. The alternative offered 

by fiber lasers in the near infrared 

region soon released this constraint: 

erbium-doped fiber laser based OFCs 

notably offer a mode-lock so reliable 

that a year of continuous operation is 

Figure 1. Architecture of atomic clocks, 

oscillators and frequency combs at SYRTE. The 

three operational OFCs act as a bridge between 

microwave and optical oscillators, probing 

respectively the SYRTE microwave fountains 

and optical lattice clocks. 

not an issue. More recently, research 

on compact combs based on micro-resonators 

or semiconductor systems 

has raised a strong interest in the community. 

Despite their low power and 

high repetition rate, making notably 

the measurement of f 0 challenging, the 

potential of devices that could be easily 

installed in a large range of academic 

or industrial applications is very appealing. 

Generally, the technical progress 

opened the door to contributions to a 

large variety of scientific and technical 

fields, way beyond frequency metrology, 

as detailed in reference [1]. Striking 

applications were for example published 

in molecular spectroscopy (dualcomb 

spectroscopy, analysis of gas 

sample to track pollution, analysis of 

human breath), astronomy (calibration 

of spectrograph), or distance measurements 

(laser ranging). 

At SYRTE (Systèmes de Référence 

Temps-Espace), the French National 

Metrology Institute for Time and 

Frequency, OFCs are used on the one 

hand for accurate and high resolution 

frequency metrology and on the 

other hand for the transfer of spectral 

purity from one frequency domain to 

another. In the following sections, we 

describe how this is applied to atomic 

clocks comparisons, to the construction 

of very low noise sources in the 

optical and microwave domains and 

to the dissemination of frequency standards 

on national and even international 

distances. 

ACCURATE AND HIGH RESOLUTION 

ATOMIC CLOCK COMPARISONS 

In the last 30 years, the SYRTE 

teams have developed and improved 

an ensemble of seven atomic 



FOCUS 

Figure 2. Spectral coverage of the SYRTE combs architecture. All the relevant laser sources 

can be counted by at least two of the three OFCs. The connection to the 1542 nm laser 

is essential for the referencing of the two Er combs and of the REFIMEVE network. 

clocks strongly contributing to the accuracy 

of international timescales UTC 

(Coordinated Universal Time) and TAI 

(Temps Atomique International), and 

to the SI second monthly calculated 

by the BIPM (Bureau International des 

Poids et Mesures). This clock ensemble 

includes four microwave fountains, 

three based on cesium and one on rubidium, 

all featuring an accuracy of a 

few 10 -16 and regularly contributing to 

the steering of TAI. In addition, we are 

developing three optical lattice clocks 

operated with strontium or mercury 

that reach an uncertainty of a few 

10 -17 and already provided calibrations 

reports to the BIPM. This is one of the 

most complete sets of clocks existing in 

the world, and in order to compare frequently 

all these instruments, we have 

developed an ensemble of three operational 

OFCs. The goal of this redundancy 

is twofold: ensure the accuracy of the 

measurements, and evaluate the stability 

of the comparisons (figure 1) 

Our architecture relies on two Erbium 

doped fiber laser based OFCs (Menlo 

Systems, FC1500) featuring a spectrum 

in the near infrared, spanning from 

1 µm to 2 µm after broadening. This 

allows a straightforward connection to 

the telecommunication bands around 

1.5 µm, but does not permit to address 

directly all optical clock transitions 

(typically between 200 and 800 nm). 

Modules doubling the frequency of the 

combs in specific spectral windows are 

therefore necessary, for instance to 

generate teeth at 698 nm and 813 nm 

to connect respectively the strontium 

clock and lattice lasers (figures 2 and 3). 

This is complemented by a former generation 

titanium-sapphire laser based 

OFC, with a spectrum after broadening 

(500 nm to 1.1 µm) enabling the 

direct comparison of all SYRTE optical 

clocks. We chose to phase-lock all our 

frequency combs to a cw ultrastable 

laser (frequency instability below 

10 -15 between 0.1 and 1000 s timescales) 

in order to reach the so-called narrow 

linewidth regime: this ultrastability is 

transferred to each tooth of the comb, 

which leads to beatnotes with external 

oscillators that can be filtered in a narrow 

band. After mixing out f 0 from all the 

beatnotes f osc , we effectively generate a 

virtual comb at f 0 = 0 after isolating the RF 

single tone at ν osc - n osc f rep , thus rendering 

every following measurement insensitive 

to f 0 . From this point, phase-locking 

a single beatnote between the comb and 

an ultrastable laser by applying feedback 

to the optical length of the femtosecond 

laser cavity, so as to keep f osc - n osc f rep 

constant, is sufficient to ‘freeze’ entirely 

f rep and therefore the position of the 

comb in frequency domain. Comparing 

the frequency of the resulting f rep signal 

(in the RF or microwave domain and 

easily obtained by photodetection of 

the pulse train emitted by the femtosecond 

laser) with signals referenced to 

microwave frequency standards 


FOCUS 


(atomic fountain clocks in particular) 

and to the SI second produces a 

high-precision measurement of the 

absolute frequency of optical clocks. 

Furthermore, by appropriate 

arithmetic combination of the various 

signals involved, it is even possible 

to extract directly the frequency 

ratio between two optical clocks without 

involvement of microwave standards 

and even independent of the 

residual noise of the OFC itself! [2] 

REFIMEVE FIBER NETWORK 

AND DISSEMINATION OF AN 

ACCURATE 1542 NM REFERENCE 

The research infrastructure 

REFIMEVE 1 is a network of optical 

fibers dedicated to the ultra-low noise 

dissemination of an ultrastable carrier 

at 1542 nm throughout the French 

territory. It provides sustainably and 

reliably (uptime > 85%) this infrared 

reference to many academic institutions 

and other research infrastructures, 

state agencies and industrials. 

The source of the disseminated signal 

is a SYRTE 1542 nm ultrastable laser, 

which defines the stability and the 

accuracy received by the end users. 

Both operational Erbium combs are 

phase locked to this laser, which in 

turns allows us to lock its frequency 

to a hydrogen maser on long (>100 s) 

timescales, while benefiting from the 

stability of a high finesse Fabry-Perot 

cavity on shorter timescales. The active 

compensation of the propagation 

noise in the network yields a signal at 

the disposal of the users featuring a 

stability below 2 × 10 -15 at any timescale, 

and an accuracy of 1 × 10 -14 on 

the fly, that can be pushed down to 

< 1 × 10 -15 on demand. 

International connections to similar 

networks have enabled comparisons 

on a continental scale between 

SYRTE and PTB (Germany, since 

2015), NPL (UK, since 2016), INRIM 

(Italy, since 2020) and soon CERN 

(Switzerland). All together, this is an 

ensemble of ~12 optical clocks that are 

interfaced with this European 1542 nm 

reference by OFCs. These instruments 

are frequently compared one to the 

Figure 3. Operational OFC ‘Er2’ at SYRTE. The 

comb is equipped with optical setups to form 

ultrastable optical beatnotes with various 

lasers. At the forefront, an electronic setup is 

being set up to generate operational low-noise 

microwave to replace the cryogenic sapphire 

oscillator in the long run. 

others, in order to ascertain the reproducibility 

of frequency ratios between 

the various contenders to a new definition 

of the second. The long term 

monitoring of these ratios also contributes 

to tests of fundamental Physics, 

such as the search for a possible drift 

of fundamental constants. Finally, 

in the last years, the field of Earth 

Sciences has expressed a growing 

interest in optical clocks: as their frequency 

is sensitive to the local gravitational 

potential, this allows them to 

contribute to the refined mapping of 

the geopotential and to an improved 

determination of the geoid (equipotential 

approximating the mean sea 

level). It is in this spirit that SYRTE 

started recently the development of a 

transportable ytterbium optical lattice 

clock, that will be moved in the future 

along the REFIMEVE 1 network and 

remotely compared to the stationary 

European clocks. This new device will 

be equipped with a transportable OFC 

in order to measure the frequency of 

the clock versus the 1542 nm carrier, 

thus opening the possibility to exploit 

the ~60 outputs of the network over 

the metropolitan territory. 

1 

Network under supervision of Université Sorbonne 

Paris Nord, Observatoire de Paris-PSL and CNRS 

TRANSFER OF 

SPECTRAL PURITY 

Beyond their measurement capacity, 

OFCs can also be utilized for novel 

applications where the excellent 

spectral purity of a state-of-the-art 

optical oscillator (cw laser) serves as 

a reference to create an ultra-pure 

radiation in an entirely different 

part of the electromagnetic spectrum, 

where high purity sources 

are difficult or even impossible 

to realize by any other technique. 

The idea is simply that when appropriately 

phase-locked to a high 

spectral purity source (in the optical 

or near-infrared domain), each 

tooth of the comb reproduces this 

spectral purity, and can be used to 

generate and/or phase-lock other 

sources in different spectral domain. 

Provided the phase comparison/ 

phase-locking processes are set up 

with extreme care and expertise to 

be sufficiently low noise, the final radiation 

will reproduce the spectral 

purity of the initial state-of-the-art 

reference, but transferred to a different 

spectral domain. 

At SYRTE, we have first demonstrated 

the use of this technique to 

transfer the spectral purity from 

an ultra-stable laser in the near-infrared 

domain (1542 nm) to an other 

laser in the same domain but at a 

significantly different wavelength 

(1062 nm) [3]. We showed how, with 

extreme precautions, this could be 

realized with a minute added noise 

corresponding to a few 10 -18 at a 1s 

timescale, well below the residual 

frequency fluctuations of any existing 

ultra-stable laser. In this case, 

the idea is simply to directly phaselock 

the 1062 nm laser onto an existing 

tooth of the comb close to it in 

frequency. Significant care must, 

however, be taken to minimize or 

cancel the noise resulting from the 

propagation of the laser radiations 

in fibered or free-space optics where 

even minute fluctuations arising 

- for example - from temperature 

changes or air flow currents may 

degrade the final performance. 



FOCUS 

In collaboration with our colleagues 

from LPL (Laboratoire de Physique des 

Lasers, CNRS/Université Sorbonne Paris 

Nord), we have also demonstrated a 

transfer of spectral purity from a 1542 nm 

wavelength laser to a ~10 µm wavelength 

radiation emitted by a Quantum Cascade 

Laser (QCL) [4]. In this case, an extra level 

of difficulty arose from the physical 

separations between SYRTE (hosting the 

1542 nm laser) and LPL (where the QCL, 

the OFC, the equipment to phase lock 

it and characterize its spectral purity, 

was residing). This difficulty was solved 

by the use of the REFIMEVE network 

which connects the two laboratories. In 

this case, the 10 µm wavelength spectral 

range is not directly accessible by existing 

OFCs, and we utilized non-linear optics 

(difference frequency generation) to 

generate a comparison signal between 

the near-infrared OFC and the 10 µm 

QCL. This technique demonstrated not 

only the most spectrally pure QCL-laser 

emission ever produced, but also provided 

absolute and SI-traceable referencing 

of its frequency. 

Last, in collaboration with our industrial 

partners from MenloSystems 

GmBH and Discovery Semiconductor 

Inc., we have demonstrated how realizing 

the transfer of spectral purity 

from a near-infrared laser (1542 nm) 

to the microwave domain (12 GHz) was 

able to produce the lowest phase-noise 

microwave signal ever demonstrated 

by any existing technique [5]. This is of 

large interest – in particular – for radar 

applications where the quest for lowphase 

noise carrier is one of the sources 

of resolution improvement. In this case, 

the microwave signal is simply produced 

by photo-detecting the train of pulses 

emitted by the OFC. Mathematically 

speaking, if the spectral purity transfer 

is perfect, since the emitted microwave 

signal is phase-coherent with the comb 

and the near-infrared reference laser, its 

phase fluctuations are imposed by those 

of the reference laser at 1542 nm, but 

with a very large division factor (of the 

order of 17000... the ratio between the 

optical and the microwave frequency). 

In order to be as close as possible to this 

mathematical ideal, an ultra-low-noise 

OFC was developed for this purpose by 

MenloSystems, and a very high linearity 

photo-diode was developed by Discovery 

Semiconductor. This is paramount for 

extreme performance, in particular 

because amplitude-fluctuations will 

generate phase fluctuations in the photo-detection 

process (even though we use 

special “magic point” conditions in the 

vicinity of which this effect is strongly reduced), 

and because high optical power 

needs to be used for photodetection in 

order to minimize the effect of thermal 

and quantum noise. We demonstrated 

an absolute phase noise of -173 dBc/Hz 

at 10 kHz and -106 dBc/Hz at 1 Hz from a 

12 GHz carrier. 

CONCLUSION 

We have presented how optical frequency 

combs, both utilized as measurement 

tools and as mean for transferring spectral 

purity between various spectral domains, 

can achieve an extremely high level of metrological 

performance and be put to use 

in the most demanding high-precision 

measurements apparatus. Extreme care 

must however be taken in order to achieve 

the best result, but as commercially available 

systems are constantly improving, 

such performances will progressively 

become available for non specialists in 

turn-key systems. 

REFERENCES 

[1] T. Fortier and E. Baumann, Commun. Phys. 2, 153 (2019) 

[2] H. R. Telle, B. Lipphardt, and J. Stenger, Appl. Phys. B 74, 1–6 (2002) 

[3] D. Nicolodi et al., Nat. Photonics 8, 219 (2014) 

[4] B. Argence et al., Nat. Photonics 9, 456 (2015) 

[5] X. Xie et al., Nat. Photonics 11, 44 (2017) 


FOCUS 


KERR FREQUENCY COMBS: 

A MILLION WAYS TO FIT LIGHT 

PULSES INTO TINY RINGS 

/////////////////////////////////////////////////////////////////////////////////////////////////// 

Aurélien COILLET 1 *, Shuangyou ZHANG 2 , Pascal DEL’HAYE 2,3 

1 

Laboratoire Interdisciplinaire Carnot de Bourgogne, 21000, Dijon, France 

2 

Max Planck Institute for the Science of Light, 91058, Erlangen, Germany 

3 

Department of Physics, Friedrich Alexander University Erlangen-Nuremberg, 91058, Erlangen, Germany 

* aurelien.coillet@u-bourgogne.fr 

Frequency combs can be generated 

in millimeter-sized optical 

resonators thanks to their ability 

to store extremely high light intensities 

and the nonlinearity of 

their materials. New frequencies 

are generated through a cascaded 

parametric amplification process 

which can result in various optical 

waveforms, from ultrastable pulse patterns to optical chaos. These Kerr frequency combs 

have been studied extensively, with a wealth of fascinating nonlinear dynamics reported, and 

myriads of applications being developed, ranging from precision spectroscopy and Lidars 

to telecom channel generators. 


Kerr frequency combs 

are a novel way to 

generate frequency 

combs based on 

the nonlinear interaction 

between 

a single wavelength light source 

and a dielectric material [1]. Since 

such interactions are quite weak, 

a resonator which forces light to 

recirculate in the material is used, 

accumulating sufficient optical 

power until new frequencies are 

generated through a nonlinear process 

called four-wave mixing (FWM, 

see insert). Depending on the parameters 

of both the resonator and 

pump light used, a variety of optical 

patterns can be generated and used 

for different applications ranging 

from time-and-frequency metrology 

to building blocks for integrated 

photonic circuits. The temporal dynamics 

of the combs generation in 

microresonators can be described 

by the Lugiato-Lefever equation (a 

nonlinear Schrödinger equation), 

which predicts exceptionally well 

the dynamics of these systems. 

RECIPE FOR A KERR 

FREQUENCY COMB 

The first and most important ingredient 

for Kerr comb generation is 

undoubtedly the resonator itself. 

It needs to have a relatively large 



FOCUS 

Depending on its exact frequency compared to the 

resonance, the pump laser light will be more or less 

enhanced, and all of the newly generated frequencies 

will interact differently with the resonances of the 

resonator, leading to a wide variety of circulating 

soliton pulse patterns. 

Kerr nonlinearity, but most importantly, 

extremely low losses, so that light 

can remain trapped for a long time. 

Typically, its quality factor Q — which is 

proportional to the lifetime of a photon 

in the cavity — has to be larger than 10 6 

to cross the threshold for parametric 

oscillations at reasonably low optical 

power. The next step consists in coupling 

as much monochromatic light 

as possible into the resonator using 

the evanescent field of a microfiber, 

cleaved fiber, waveguide or a prism, 

and scan the input frequency to excite 

one resonance. When the pump 

light within the resonator exceeds the 

threshold for comb generation, new 

frequencies are generated. The generated 

comb (corresponding to solitons 

in the time domain) is coupled out of 

the resonator for further analysis or 

for use in applications. Depending 

on its exact frequency compared to 

the resonance, the pump laser light 

will be more or less enhanced, and 

all of the newly generated frequencies 

will interact differently with the 

resonances of the resonator, leading 

to a wide variety of circulating soliton 

pulse patterns. 

Four-wave mixing is one of the effects of the third-order Kerr optical 

nonlinearity: assuming two optical waves with different frequencies 

ν 1 and ν 2 are co-propagating inside a Kerr material, a modulation occurs which 

leads to the creation of two new frequencies ν 3 and ν 4 such that energy is 

preserved, that is ν 1 + ν 2 = ν 3 + ν 4 . One can also use one single pump wavelength, 

as is the case for Kerr frequency comb generation, and such process is then 

called degenerate four-wave mixing. Once new frequencies have been 

generated though, a cascading effect can occur and all the spectral lines can 

interact through four-wave mixing. Since the Kerr effect is rather low in most 

materials, a high power is required for this conversion to be efficient. In the 

case of Kerr frequency comb generation, such high power is reached thanks 

to the cavity enhancement of the optical field at resonance. Like most Kerr 

effects, four-wave mixing is phase-sensitive, hence leading to inherently phaselocked 

frequency lines. 


FOCUS 


THE LUGIATO-LEFEVER 

EQUATION 

The initial observation of various 

types of Kerr combs led to the 

search for an appropriate theoretical 

model that could describe the 

experimental observations. One can 

understand Kerr comb generation 

either by looking at it as a collection 

of spectral modes that interact 

through four-wave mixing [2], or 

rather like the propagation of light 

in a closed-loop, with a continuous 

pump wave constantly pouring energy 

into the system [3]. In both of 

these approaches, one finds that the 

optical field in the cavity follows the 

following normalized equation: 

ψ 

— τ 

= –(1 + iα)ψ + i|ψ| 2 ψ – i β – 

2 

2 ψ 

— 

θ 

2 + F 

The evolution of the field is hence 

governed by the interactions 

between losses, the frequency 

detuning between the pump and 

the resonance α, the normalized 

nonlinearity, the second order 

dispersion of the resonator β and 

the pump field amplitude F. With 

Figure 1. a) Simplest experimental scheme for Kerr comb generation with an amplified, 

single-wavelength laser source coupled to a resonator using a microfiber in this case. The 

output signal can directly be monitored through the same coupling microfiber. b) Examples 

of different Kerr combs (light and dark blue) generated in the same resonance, at the same 

power, but for a slightly different detuning between the cavity resonance and the pump 

frequency. 

these notations, the field ψ depends 

both on the long timescale 

time τ and the angle θ which marks 

the position within the resonator. 

This equation is a modified version 

of a nonlinear Schrödinger 

equation, with both losses and 

driving included, and is known as 

the Lugiato-Lefever equation. This 

equation was introduced in 1987 by 

Luigi Lugiato and René Lefever [4] 

in order to model the spontaneous 

formation of patterns in 2D resonant 

Kerr media, and a large corpus 

of theoretical analyses is already 

available. In particular, the analysis 

of the stability of the flat solution, 

that is when no other frequencies 

or combs are generated, predicts 

in which region of the parameter 

space interesting structures can 

be formed [5]. For our purpose, a 

Figure 2. a) Bifurcation diagram of the 1D Lugiato-Lefever equation in the anomalous 

dispersion regime (β < 0) with the regime of Kerr comb one can expect in each region of 

the parameter space. The detuning α is the difference between the laser and resonance 

frequency normalized by the resonance bandwidth. b) Numerical simulations of the 

intracavity intensity for the most notable regimes of Kerr comb. Turing rolls and soliton 

are stable structures, while breather and chaotic regimes vary with time. c) Comparison 

between experiments and the LLE model for the spectra of the generated Kerr combs, while 

varying the detuning along the green line in a). 



FOCUS 

resonator has its dispersion fixed during the manufacturing 

process, and the only 2 parameters that 

remain accessible to the experimentalist are the detuning 

of the laser with respect to the resonance frequency 

α and the pump power F 2 . In the anomalous 

dispersion regime, a wealth of comb states can be 

achieved: Turing rolls appear spontaneously above 

a given threshold, and correspond to sine-like oscillation 

of the intensity. A soliton is obtained when the 

laser frequency is slightly higher than the resonance 

frequency (positive detuning), but requires a seed 

pulse or significant fluctuations to be generated, 

as this part of the parameter space is multi-stable; 

multiple excitations can therefore lead to multiple 

solitons. Increasing the pump power can lead to instability, 

with breathers and chaotic pulsed regimes 

taking place. Despite its mathematical simplicity, the 

Lugiato-Lefever has been able to model the experimental 

results obtained with various resonators with 

an impressive precision, as can be seen in figure 2 

c). The evolution from a stable comb to chaos is reproduced 

with high accuracy over several orders of 

magnitude of intracavity power. Such an accuracy 

stems from the very simple processes at play, and 

particularly the parametric gain: the Kerr effect is 

indeed quasi-instantaneous, with a very large spectral 

bandwidth, such that its modelling is very easy. 

BEYOND THE LUGIATO-LEFEVER EQUATION 

Rich cavity soliton states have been predicted by 

Lugiato-Lefever equation and experimentally observed 

in different dispersion regimes and with different 

pumping schemes. As shown in Fig. 3, with a single frequency 

pumping and anomalous dispersion, bright soliton 

pulses (intensity peaks on a dark background) can 

be generated through a well-defined balance between 

the Kerr nonlinearity, group velocity dispersion, cavity 

loss and gain [6]. Crystallized soliton structures 

with crystallographic defects have also been observed 

in this regime. These correspond to fixed patterns 

of solitons that are stable within the resonator and 

repeat after each round-trip. In contrast, the normal 

dispersion regime leads to dark pulses (intensity dips 

embedded in a high-intensity background) that arise 

through the interlocking of switching waves connecting 

the homogeneous steady states of the bi-stable 

cavity solutions (middle panel in the right of Fig. 3) 

[7]. Distinct soliton structures in the zero-dispersion 

regime have also been reported with asymmetrical 

behaviour both in the frequency domain and time domain. 

The zero-dispersion regime is of particular interest 

because it allows to investigate how higher-order 

dispersion affects the soliton formation dynamics. The 

soliton (bright or dark) formation in this regime is associated 

with the interlocking of abrupt changes of 


FOCUS 


Figure 3. Different light pulses emitted from the resonators. With single frequency driving 

of a resonator (marked with red boxes), bright solitons and soliton crystals can be generated 

in anomalous dispersion regime, while dark pulses can be observed by driving at normal 

dispersion. Seeding with two colors of lights, dark-bright soliton bound states can be 

emitted from a resonator with a mutually trapped dark-bright soliton pair. 

power in a resonator, so-called 

switching waves. Most importantly, 

zero or small dispersion is very desirable 

to obtain spectrally broadband 

frequency combs. In addition, the 

coexistence and mutual locking of 

dark and bright pulses in the regime 

of normal, zero, and anomalous 

dispersion has been predicted by 

the Lugiato-Lefever equation when 

considering higher-order dispersion 

effects. Recent research revealed 

bound states of dark-bright solitons 

in a resonator through seeding two 

modes with opposite dispersion via 

two colors of light (lower panel in 

the right of Fig. 3) [8]. These mutually 

trapped dark-bright pulses lead 

to a light state with constant output 

power in the time domain but spectrally 

resembling frequency combs. 

In addition to the generation by a 

CW laser source, cavity solitons can 

be generated with even higher efficiency 

by synchronously pulsed-driving. 

The corresponding soliton 

dynamics are also well modelled 

by the Lugiato-Lefever equation. 

An additional way for the generation 

of Kerr solitons is the use of 

active media as part of an external 

cavity, which has been studied with 

increasing attention in the last years. 

This concept has the advantage of not 

requiring a narrowband tuneable laser 

source for the soliton generation. 

CONCLUSION 

Frequency combs generated through 

four-wave mixing in high quality 

factor resonators constitute a very 

promising alternative to modelocked 

lasers for many applications 

where compacity is required. In the 

past 15 years, research on this topic 

went from early demonstration of 

frequency generation to theoretical 

REFERENCES 

understanding and prediction, and 

to applications, including optical 

frequency synthesis, ultrastable 

microwave generation, calibration 

for astronomy spectroscopy, and 

ranging. Most of these applications 

make use of a single pulse revolving 

inside the cavity – a unique soliton 

– yet a myriad of other exotic and 

physic-rich regimes can be obtained. 

The Lugiato-Lefever equation and its 

refinements have been used to accurately 

predict the generation of these 

original states, such as soliton crystals 

and locked switching waves. The 

full extent of the possibility offered 

by Kerr frequency combs and their 

variants – pulse-driven, dual-pump, 

with gain medium, … – remains to 

be explored, both for fundamental 

study in nonlinear dynamics and for 

demanding applications. 

[1] P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth & T. J. Kippenberg, Nature 

450, 1214-1217 (2007) 

[2] Y.K. Chembo, & C. R. Menyuk, Phys. Rev. A 87, 053852 (2013) 

[3] S. Coen, H. G. Randle, T. Sylvestre, & M. Erkintalo, Opt. Lett. 38, 37-39 (2013) 

[4] L.A. Lugiato, & R. Lefever, Phys. Rev. Lett. 58, 2209 (1987) 

[5] C. Godey, I. V. Balakireva, A. Coillet, & Y. K. Chembo, Phys. Rev. A 89, 063814 (2014) 

[6] T. Herr, V. Brasch, J. D. Jost et al., Nature Photon. 8, 145–152 (2014) 

[7] X. Xue, Y. Xuan, Y. Liu et al., Nature Photon. 9, 594–600 (2015) 

[8] S. Zhang, T. Bi, G. N. Ghalanos, N. P. Moroney, L. Del Bino, & P. Del’Haye, Phys. Rev. Lett. 

128, 033901 (2022) 



optical helicity 

THE OPTICAL HELICITY 

IN A MORE ALGEBRAIC APPROACH 

TO ELECTROMAGNETISM 

/////////////////////////////////////////////////////////////////////////////////////////////////// 

Ivan FERNANDEZ-CORBATON* 

Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany 

*ivan.fernandez-corbaton@kit.edu 


The constant miniaturization trend in 

nanophotonics challenges some of the 

theoretical tools at our disposal: Analytical 

shortcuts such as the dipolar and paraxial 

approximation become unapplicable, and 

the computational cost of fully numerical 

studies often renders them impractical. 

A basic electromagnetic property, the optical 

helicity, and the use of more abstract tools 

inspired by it, are rising to meet the challenge. 

The optical helicity, also known as electromagnetic 

helicity, represents the handedness of Maxwell 

fields, and is the conserved quantity connected 

to the electromagnetic duality symmetry. This 

connection allows to consider the polarization of 

the field within the powerful framework of symmetries 

and conservation laws. Such framework is formalized 

in electromagnetic Hilbert spaces thanks to the existence of an 

appropriate scalar product for Maxwell fields [1]. While these 

ideas may seem of purely theoretical interest, the abstraction 

and generality that they afford facilitate both the understanding 

and the design of specific light-matter interaction effects. 

The algebraic treatment of helicity and its eigenstates [2] is 

also a valuable tool for extending the theory, as exemplified 

by the recently established new connection between optics 

and magnetism [3]. 

FUNDAMENTALS AND APPLICATIONS 

Helicity is the projection of the angular momentum vector J 

onto the direction of the linear momentum vector P, 

Λ = — J • P . (1) 

|P| 

Equation 1 is the most general form of the helicity operator, 

which is valid for many particles and fields, such as electrons 

and gravitational waves. For Maxwell fields, helicity 

describes the sense of screw in light: The circular polarization 

handedness. 

Helicity is a pseudoscalar: Its sign flips under any spatial 

inversion operation such as parity or mirror reflections. 

Figure 1 illustrates some transformation properties 

of helicity comparing them with the transformation 

properties of angular momentum. Helicity and angular 

momentum are two different properties of the field. A 

very basic difference is that, while a two dimensional 

plane is enough to define a rotation (angular momentum), 

three spatial dimensions are required to define a 

sense of screw (helicity). The systematic consideration 

of how helicity and angular momentum transform allows 

the identification of the symmetry reasons for particular 

light-matter interaction effects, which can then be used 

for the analysis and prediction of experimental measurements. 

For example, the optical vortices seen in focusing 

by or scattering off cylindrically symmetric systems are 

readily shown to be due to the breaking of a different 




In its most general illuminationindependent 

embodiment, helicity 

preservation is achieved by objects 

exhibiting electromagnetic duality 

symmetry, a fundamental continuous 

symmetry in electromagnetism. 

symmetry in each case (see Chap. 3 [4]): Translational 

symmetry in focusing, and electromagnetic duality symmetry 

(see below) in scattering. For the directional coupling 

of emitters onto waveguides, it is readily shown 

that the polarization handedness of the emission cannot 

be responsible for the directionality, which is controlled 

by angular momentum [5], see Fig. 3. 

For transverse Maxwell fields, helicity has two possible eigenvalues, 

λ = +1 and λ = -1, whose corresponding eigenstates for 

(r,t) -dependent Maxwell fields are (SI units assumed): 

F λ (r,t) = √ 

– 0 

—2 [E(r,t) + iλc 0 B(r,t)], ΛF λ (r,t) = λF λ (r,t). (2) 

The F λ (r,t) can be built as the following sum of plane waves: 

dk 

F λ (r,t) = ∫ 

3 - {0} 

√ — F λ (k) exp(ik • r - ic 0 |k|t), (3) 

(2π) 3 

where k • F λ (k) = 0, i ^k × F λ (k) = λF λ (k) since Λ i ^k× in this 

representation and, importantly, the frequency is restricted 

to positive values ω = c 0 |k| > 0. The F λ (r, t) are the positive frequency 

restriction of the Riemann-Silberstein vectors 

Figure 1. Transformations of helicity, represented by the 

helices, and of angular momentum, represented by the 2D 

turns. The initial objects in a) are transformed by a rotation 

by 180 degrees along the y axis in b), by the parity operation 

in c), by a mirror reflection across the YZ plane in d), and by 

a mirror reflection across the XY plane in e). Spatial inversion 

transformations always flip the screw sense of the helix, while 

rotations never do. The chosen angular momentum changes 

sign (the sense of the turn is inverted) upon the rotation in b) 

and the reflection in e), and stays invariant upon parity and the 

reflection in d). 




Figure 2. F± split a general 

electromagnetic field such 

as the arbitrary multifrequency 

combination 

of circularly polarized 

plane waves in a) into its 

two helicity components, 

left-handed (blue) in b), and 

right-handed (red) in c). 

[2], and its monochromatic components are also known as 

Beltrami fields [6]. As illustrated in Fig. 2, the F λ (r, t) split 

the electromagnetic field into its left and right circular polarization 

handedness, for λ = +1 and λ = -1, respectively. The 

restriction to positive frequencies makes E(r, t) and B(r, t) 

necessarily complex valued, and is crucial for achieving the 

handedness splitting: If E(r t)and B(r, t) are real-valued, then 

|F + (r, t)| = |F–(r, t)|follows from Eq. 2, negating the handedness 

separation. 

The splitting allows to analyze the handedness of near 

fields around illuminated nanostructures, such as the silicon 

disk in Fig.4, which is illuminated on axis by a single plane 

wave of positive helicity. Figures 4a) and 4b) correspond to 

illumination with two different frequencies. Each sub-figure 

is divided into two areas where the false color scale shows 

point-wise intensities: |F + (r,|k|)| 2 on the left, and |F–(r,|k|)| 2 

on the right. The monochromatic F λ (r,|k|) are obtained as 

in Eq. 2, but using single frequency complex electric and 

magnetic fields. To a good approximation, and in contrast 

with what Fig. 4b) shows, Fig. 4a) shows that the disk does 

not couple the two helicities upon light-matter interaction: 

It preserves the incident helicity. In its most general illumination-independent 

embodiment, helicity preservation 

is achieved by objects exhibiting electromagnetic duality 

symmetry, a fundamental continuous symmetry in electromagnetism, 

whose action is: 

E(r,t) → E θ (r,t) = E(r,t)cosθ - c 0 B(r,t)sinθ, 

c 0 B(r,t) → c 0 B θ (r,t) = E(r,t)sinθ + c 0 B(r,t)cosθ, 

for a real angle θ, and its action on the helical fields 

(4) 

F λ (r,t) → F θ λ (r,t) = F λ(r,t) exp(–λiθ) (5) 

reveals that, in the same way that rotations are generated by 

angular momentum operators, e.g. R z (θ) = exp(–iθJ z ), duality 

is generated by the helicity operator D θ = exp(–iθΛ). The polarization 

of the field can then be treated within the powerful 

framework of symmetries and conservation laws. Such 

Figure 3. Emitters (circles) on top of waveguides (gray boxes) emit light with a definite angular momentum along the z-axis 

(yellow-brown arrows), and a definite helicity (blue/red circles), which couples asymmetrically to the waveguide modes, with more 

power going towards the +x direction than to the -x direction, or vice versa (purple arrows). The initial situations [panels a) and e)] 

are transformed by mirror reflections which are symmetries of the waveguide and leave the position of the emitter unchanged: 

Mx [panels b) and f)], Mz [panels c) and g)], and the composition MxMz [panels d) and h)], respectively, where α indicates a 

reflection across the plane perpendicular to the α direction. In each panel, the origin of coordinates is at the position of the emitter, 

and the coordinate axes are oriented as shown in the central part of the figure. The transformation properties of helicity allow us 

to conclude that it cannot be responsible for the directional coupling: For example, e) and h) show the same helicity producing 

the opposite directionality. Such contradictions do not arise for angular momentum, which is indeed the property of light which 

controls directionality [5]. 




framework facilitates the identification of design guidelines 

for achieving specific effects, such as zero back-scattering 

(anti-reflection) (see Chap. 4 in [4]), and enhanced near-field 

interaction with chiral molecules (see Chap.5.3 in [4], [7]). 

Helicity preservation is one of the guidelines in these two 

particular cases. Unfortunately, dual-symmetric systems are 

hard to fabricate. The theoretically most straightforward 

way to design a dual system is by using materials with equal 

relative electric permittivity and magnetic permeability: a 

very demanding requirement. Fortunately, the essential 

idea in duality symmetry, balanced electric and magnetic 

responses, allows to design non-magnetic systems that preserve 

helicity, at least for particular frequencies and illumination 

directions. For example, the aspect ratio of the silicon 

disk in Fig. 4 was optimized [7] for hosting equal electric 

and magnetic dipole moments upon on-axis illumination 

at f = 125 THz. 

A SCALAR PRODUCT PROVIDES MORE TOOLS 

The appropriate scalar product for Maxwell fields is [1]: 

F|G 

dk 

= ∫ 

3 — 

- {0} 

ħc 0 |k| [F + (k)† G + (k) + F–(k) † G–(k)] (6) 

where |F and |G are two different sets of Maxwell fields, specified 

by their corresponding F λ (k), and G λ (k). Equation 6 is 

conformally invariant: Its numerical value is identical to the 

scalar product between X|F and X|G, for any transformation 

X in the conformal group, the 15 parameter group which is the 

largest group of invariance of Maxwell equations [8]. 

Equation 6 is defined for free Maxwell fields which do not 

interact with matter. Nevertheless, in a light-matter interaction 

sequence such as the one depicted in Fig.5a), Eq. 6 can be applied 

to the pre- and post-interaction fields, only excluding the 

grayed out period where the interaction is ongoing. 

The scalar product for Maxwell fields allows to use the tools 

of Hilbert spaces in electromagnetism, and to leverage 

Figure 4. Helical decomposition of the scattered field around 

a silicon disk illuminated on axis with two different frequencies 

(adapted from [7]). 




it produces is longitudinal (parallel to k), and is hence annihilated 

by the action of the helicity operator Λ i^k×. Importantly, 

Λ ω=0 can be readily shown to be 1/2√ — 0 /µ 0 times the magnetic 

helicity. The electromagnetic helicity of the dynamic Maxwell 

fields and the static magnetic helicity [9] are unified as two different 

embodiments of the same physical quantity, establishing 

the theoretical basis for studying the conversion between the 

two embodiments of total helicity upon light-matter interaction 

[3], as illustrated in Fig.5. 

Figure 5. Helicity exchange between electromagnetic fields 

and material magnetization. In a) a beam of positive helicity 

interacts with a magnetization of the opposite helicity and flips 

its handedness. In b), an initially chiral magnetization is turned 

into an achiral one by achiral and non-necessarily optical 

forces. In this process, the material radiates an electromagnetic 

field which contains (part of) the helicity lost by the material. 

some of the methods from quantum mechanics. Given an electromagnetic 

field |F, the average of its linear momenta, angular 

momenta, energy, and any other quantity represented by 

a hermitian operator Γ is F|Γ|F, which can be readily written 

down as a k-space integral [2] using Eq.6. 

Recently, the k-space integral expression of the average 

electromagnetic helicity, F|Λ|F, has lead to the definition 

of material helicity [3]. Matter in static equilibrium is electromagnetically 

represented by its electric charge density 

ρ(r), and its magnetization density M(r), which are bijectively 

connected to the static fields that they generate at ω = 0. It is 

then possible to define the total helicity, Λ, as the sum of 

two terms: The electromagnetic helicity, F|Λ|F = Λ ω>0 that 

measures the difference between the number of left-handed 

and right-handed photons of the free field, and a static term, 

Λ ω=0 , that measures the screwiness of the static magnetization 

density in matter: 

Λ = Λ ω>0 + Λ ω=0 , where 

Λ ω>0 dk 

= ∫ 

3 — 

- {0} 

ħc 0 |k| |F + (c 0|k|, k)| 2 – |F–(c 0 |k|, k)| 2 , 

Λ ω=0 dk |M 

= ∫ 

3 — + (0, k)| 2 – |M–(0, k)| 

—— 

2 

- {0} 

ħc 0 |k| 

2/µ0 

The F ± (c 0 |k|, k) are the F λ (k) of Eq.3, and M λ (0, k) 

are the λ = ±1 helicity components of the 3D Fourier 

transform M(0, k) of the static magnetization density 

M(r): 2M λ (0, k) = (I + λ i^k ×) (i^k ×) 2 M(0, k). The charge density ρ(r) 

does not host any material helicity because the electric field that 

(6) 

OUTLOOK 

The design of helicity preserving systems is an area of research 

with technological implications. While approximations such 

as the geometric optimization illustrated in Fig.4 can already 

be of some use, recent advances in molecular science prompt 

the question of whether a truly dual symmetric material, albeit 

likely with a narrow operational frequency band, can be 

achieved through molecular design. 

Finally, the transfer of helicity from the field into a magnetic 

material is a mechanism for affecting the chirality of matter 

in static equilibrium, that is, in its ground state. This points 

to possible applications in magnetization control by optical 

means. The potential role of the effect in the all-optical helicity-dependent 

magnetization switching [10] is an exciting 

research question. 

ACKNOWLEDGMENTS 

Partially funded by the Deutsche Forschungsgemeinschaft 

(DFG, German Research Foundation) --Project-ID 258734477 

-- SFB 1173. 

REFERENCES 

[1] L. Gross, J. Math. Phys. 5, 687 (1964). 

[2] I. Bialynicki-Birula, Prog. Optics 36, 245 (1996). 

[3] I. Fernandez-Corbaton, Phys. Rev. B 103, 054406 (2021). 

[4] I. Fernandez-Corbaton, Helicity and duality symmetry 

in light matter interactions: Theory and applications, Ph.D. 

thesis, Macquarie University (2014), arXiv: 1407.4432. 

[5] A. G. Lamprianidis, X. Zambrana-Puyalto, C. Rockstuhl, 

and I. Fernandez-Corbaton, Laser & PhotonicsReviews 16, 

2000516 (2022). 

[6] Lakhtakia A., Beltrami fields in chiral media (World 

Scientific, 1994). 

[7] F. Graf, J. Feis, X. Garcia-Santiago, M. Wegener,C. Rockstuhl, 

and I. Fernandez-Corbaton, ACS Photonics 6, 482 (2019). 

[8] H. Bateman, Proceedings of the London Mathematical 

Society s2-7, 70 (1909). 

[9] M. A. Berger, Plasma Physics and Controlled Fusion 41, 

B167 (1999). 

[10] C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, 

A. Tsukamoto, A. Itoh, and T. Rasing, Phys. Rev. Lett. 99, 

047601 (2007). 





BUYER'S GUIDE 

nanopositioner solution 

HOW TO SELECT 

A NANOPOSITIONER 

SOLUTION? 

/////////////////////////////////////////////////////////////////////////////////////////////////// 

Emmanuel Pascal 1, *, Scott Jordan 2 

1 

PI FRANCE, 380 avenue Archimède, bât D, 13100 Aix en Provence, France 

2 

PI USA, Head Office, 16 Albert Street, Auburn, MA 01501, USA 

*e.pascal@pi.ws 


Many recent innovations have been enabled by high 

precision positioning devices operating at a nanometer 

level or below. Developments in this field have been 

very fast over the last years supported by applications 

in material sciences, genomics, photonics, defense, 

biophysics, and semiconductors creating challenges 

for the scientists and engineers in need of precise and 

yet robust positioning systems. The critical point is: 

how should you select your nano positioning solution? 

NANOPOSITIONING SYSTEM: 

A NEW DEFINITION? 

By its original definition, a nanopositioning 

device is a mechanism 

capable of repeatedly delivering 

motion in increments as small as one 

nanometer. Lately, demands from 

industry and research have pushed 

requirements to the picometer range. 

While electroceramics such as piezo 

materials with flexure guides remain 

the gold standard for breaking the 

resolution nanometer barrier, there 

are several other solutions available 

today providing repeatable single-digit 

nanometer step resolution including 

linear motors, voice-coil drives, 

and frictionless guides such as air 

bearings and magnetic bearings. 

Emmanuel Pascal: Country Director, 

PI France 

Scott Jordan: Head of Photonics Market 

Segment, PI Group 

Dynamic issues dominate many applications 

and can be addressed by 

novel approaches that benefit realtime/high-dynamic 

applications. 

They present new opportunities for 

optimizing process economics. 

Rapid testing and packaging of 

the latest silicon photonics (SiPh) 

devices — starting at the wafer level 

— is the perfect example. In these 

applications, optical elements and 

probes must be brought into perfect 

coupling with devices in various 

stages of manufacture from wafer to 

final package. With thousands of optical 

elements on a single wafer, coupling 

time becomes the most critical 

cost factor in testing. The same applies 

to all further process steps up 

to assembly and packaging, where 

active and passive optical elements 

(e.g., LEDs, laser diodes, photodiodes 

and optical fibers and waveguides) 

must be precisely positioned 

Figure 1. A doublesided 

test and 

alignment system 

for silicon photonics 

wafers based on 

hexapods and piezo 

nanopositioning stages 

to combine the fastest 

alignment speeds with 

the highest flexibility. 


nanopositioner solution BUYER'S GUIDE 

Figure 2. 

A motion 

amplified, 

flexure-guided 

actuator based 

on a multilayer 

piezo stack. 

Piezo flexure 

drives can 

last 100 

billion cycles! 

relative to each other, typically with accuracies 

in the sub micrometer range. 

Here, too, the time required for coupling 

optimization is decisive for cost-effectiveness. 

What makes these applications 

even more challenging is the frequent 

requirement to optimize positioning in 

up to all six degrees of freedom. This requires 

intelligent, precisely coordinated 

motion control in order not to destroy 

the result once achieved in one axis by 

movements in another axis. Complex 

algorithms that enable parallel, i.e., simultaneous, 

optimization in multiple 

degrees of freedom are the solution 

here. This creates a new paradigm for 

the definition of a nanopositioning system. 

We are not talking anymore about 

a single piezo flexure stage but sophisticated 

and yet robust systems, built application 

specific, that can work from 

lab to fab environment. 

NANO POSITIONING: 

NO FRICTION PLEASE 

As a rule of thumb, the lower the mechanical 

friction in a motion system, the 

higher the positioning performance. 

Three frictionless guiding technologies 

are often used in nanopositioning applications: 

flexures, air bearings, and 

magnetic bearings. 

Flexures guides are highly stable and 

stiff, but with travels limited to the millimeter 

region. For motion ranges from 

a few to hundreds of millimeters, air 

bearings offer a solution. An air-bearing 

stage is a linear or rotary positioner 

constrained on a cushion of air, using 

preload mechanisms, virtually eliminating 

mechanical contact and thus wear, 

friction, bearing noise, and hysteresis 

effects. Air-bearing nano-positioning 

stages are used in numerous high precision 

motion applications, such 

PIEZO ACTUATORS ON MARS! 

All-ceramic-encapsulated, cofired 

piezo actuators were introduced many 

years ago pushing performance and 

reliability to new limits to respond to the 

industrial requirements. 

These actuators improve greatly the 

reliability of devices based on this 

technology, especially when used in challenging environments. 

In fact, NASA/JPL engineers ran these actuators through harsh life tests 

including 100 billion expansion/contraction cycles before selecting them 

for the Mars Rover’s science lab! 


BUYER'S GUIDE 


as metrology, flat panel display inspection, 

large-area photonics test etc. 

The non-contact design also makes 

these stages work well in cleanroom 

applications. 

SHORT TRAVEL PIEZO 

POSITIONERS 

When talking about nanopositioning 

actuators with precision in the 

nm range and travel ranges of up 

to a few millimeters, piezo actuators 

combined with flexure designs 

are the historical solution. Flexure 

guides are frictionless and can be 

extremely small and robust at relatively 

low cost. 

Piezoelectric technologies play 

a foundational role in positioning 

applications with nanometer resolution 

requirements. The benefits of 

piezo-based devices include: 

• Unlimited resolution: positioning 

increments well below 1nm are 

possible and fit applications ranging 

from semiconductor to super 

resolution microscopy 

• Fast response (microsecond time 

constants) 

• Maintenance-free, solid-state 

construction that reduces wear 

and tear 

• High efficiency: energy is absorbed 

only to perform movement 

• Inherent vacuum-compatibility 

• Non-magnetic and magnetic-insensitive 

construction 

• High throughput and dynamic 

accuracy 

Figure 3. Example of Voice Coil Miniature Linear 

Stage with Air Bearings. 

LONG-TRAVEL 

PIEZO TECHNOLOGIES 

By combining lateral actuation 

and longitudinal actuation of piezo 

actuators, it is possible to create 

the basic element of a piezo walking 

drive. A digital controller sequences 

their operation, providing 

high-force, long travel step-mode 

actuation plus picometer resolution 

fine high-bandwidth actuation. 

Forces can be generated to 

800N and resolution down to 

50pm are achievable. This unique 

combination of class-leading characteristics 

is proving to be an enabling 

technology for a wide variety 

of applications. 

Figure 4. High-load walking drives combine 

piezo clamping and shear actuators in order 

to move a rod. 

Another use of piezo ceramics 

technology is in ultrasonic piezo 

motors. These are composed of monolithic 

slabs of piezo ceramics; standing 

waves are driven in the substrate 

at frequencies of tens to hundreds of 

kilohertz. A hardened contact-point 

attached at a resonant node-point is 

thereby made to oscillate in a quasi-elliptical 

fashion; when preloaded 

against a runner, this confers linear 

or rotary motion. These piezo motors 

achieve up to 500mm/sec or 720 

degrees/sec over virtually unlimited 

travel ranges while providing high 

precision and fast start/stop dynamics. 

A key benefit of this class of 

motor is the in-position stability. 

CLOSED LOOP OPERATION: 

SELECTING THE RIGHT SENSOR/ 

CONTROLLER SOLUTION 

Achieving nanometer precision 

requires further technologies. The 

stage’s internal metrology system 

must also be capable of measuring 

motion at the nanometer scale. The 

characteristics to consider when selecting 

a stage metrology system are 

linearity, resolution, stability and 

bandwidth. Other factors include the 

ability to measure the moving platform 

directly and contact vs. noncontact 

measurements. Three types of 

sensors are mostly used in nanopositioning 

applications: linear encoders 

(for longer travels), and capacitive 

and strain sensors (for short travels). 

Linear encoders are familiar devices 

for measuring long displacements 

up to hundreds of millimeters. 

While resolution in the nanometer 

range is common, accuracy is typically 

limited to 1 µm per 100 mm. 

However, this can be improved significantly 

with modern controllers. 

Incremental encoders measure 

changes in position and must be initially 

referenced to a home switch on 

power up; absolute encoders do not 

require this step but tend to be costlier. 

For short travel nanopositioning 

devices, cost-effective strain gauges 

(incl. piezoresistive sensors) use elements 

whose electrical characteristics 


nanopositioner solution BUYER'S GUIDE 

While resolution in the nanometer range is common, 

accuracy is typically limited to 1 µm per 100 mm. However, 

this can be improved significantly with modern controllers. 

change with strain. These devices are 

usually attached to the piezo ceramics 

itself or to a structural element of the 

stage. Special care must be taken when designing 

them into a mechanism. If there 

are elastic or frictional elements in the 

path between the point of motion and the 

point of measurement, errors will result. 

Physically small sensors (such as piezoresistive 

sensors) measure a highly spatially 

localized strain, from which the overall 

mechanism position can be inferred. 

And these sensors cannot be configured 

to compensate for orthogonal (parasitic) 

errors in multi-axis configurations — only 

parallel metrology of the actual moving 

platform can provide this valuable capability. 

Capacitive sensors have emerged 

as the preferred solution for the most demanding 

nanopositioning applications 

requiring short travel ranges. They are absolute, 

extremely accurate and ultrahighresolution 

devices for determining absolute 

position over ranges of hundreds of 

microns or even millimeters. The device’s 

positioning motions vary the distance 

between two nano-machined capacitor 

plates, providing a sensitive and drift-free 

positional feedback signal. 

ACTIVE OPTIC MOUNTS: 

TRAVELLING IN SPACE 

Beyond linear single or multi-axes configurations, 

piezoelectric devices are very suitable 

to create tip/tilt platforms. With the 

integration of actuators with resolutions 

well below 1nm, these systems routinely 

offer angular resolutions surpassing 1/100 

arcsec. Compact piezo- and voice-coil-actuated 

active optic mounts are available 

with multiple degrees of optical deflection. 

Two-axis, parallel-kinematic piezo 

mounts offer advantageous gimbal-style 

actuation with coplanar rotational axes. 

Compared to galvos and voice-coil systems, 

piezo-driven active optic mounts are 

typically more compact, fieldless, faster, 

inherently stiffer, and more predictable 

when power is cut. They are of great interest 

in the innovative field of Free Space 

Optical Communications (FSOC), where 

thousands of satellites form a high-speed, 

low-latency global information network. 

For these applications, robustness, long 

life and environmental insensitivity 

are prized. 

INTERFACING 

A wide variety of interfaces is available 

today, RS-232 to Ethernet, SPI, and USB, 

plus a host of specialty interfaces are 

among them. The choice depends on 

your environment, and all have pros 

and cons. Since motion controllers communicate 

via short messages, latency is 

often much more important to overall 

throughput than bulk transfer speed. 

USB and Ethernet are very common, 

with the latter supporting remote and 

distributed communications at the cost 

of variable latencies when the common 

TCP/IP communications protocol 

Figure 5. 

Example 

of Voice Coil 

Miniature Linear 

Stage with 

Air Bearings. 


BUYER'S GUIDE 


rapid optical alignments. Advanced 

controllers for sophisticated automation 

tasks in industrial environments 

are now available on the market with 

nanoscale capabilities. They can drive 

linear or brushless motors, are based 

on a real time architecture and serve 

applications such as laser material 

processing, photolithography, fine 

inspection or any application where 

tight synchronization is required. 

They can generate controlled trajectories—for 

example from a 3D CAD 

file—and offer advanced correction 

capabilities to achieve the best results. 

is used. By comparison, EtherCAT is 

a deterministic, open protocol that 

leverages the ubiquity and speed of 

Ethernet hardware, allowing flexible 

construction of sophisticated and 

scalable systems. Although analog 

interfacing may sound obsolete, it has 

essentially zero latency and infinite 

speed and is easily coordinated with 

data acquisition, triggering and other 

real-time processes. Good implementations 

of a digital interface with a 

precision DAC generally add internal 

waveform generation and synchronization 

capabilities; the highest-performance 

implementations are of course 

fully fledged DSP architectures which 

eliminate the analog servo-circuitry 

entirely. A particularly common application 

of nanopositioners to photonics 

is in automated alignment. 

Recent advanced controllers for 

piezo nanopositioners, hexapods 

and stage stacks embed automated 

alignment routines including parallel 

gradient search methods for the most 

Figure 6. A tip/tilt beam steering platform, 

used in the Solar Orbiter space probe. In 

principle various sizes of mirror can be installed 

and optical deflections to several degrees 

are achievable. 

VENDORS IN EUROPE 

Aerotech 

Attocube 

Cedrat Technologies 

Leuwen Air Bearings (LAV) 

Mad City Lab (MCL) 

MKS Newport 

nPoint 

Physik Instrumente (PI) 

Piezo Motor 

Piezo System Jena 

Piezoconcept 

Pro-lite 

Smaract 

Tekceleo 

Thorlabs 

CONCLUSION 

It is nowadays possible to build 

advanced instrumentation with 

nanoscale performances with various 

technologies. Beyond the fundamentals 

of product capability, 

quality, global support, and applications 

savvy, choosing a vendor represents 

a choice of partners. There are 

tangible benefits of working with a 

supplier whose experience crosses 

many fields and many drive technologies. 

Such a supplier is able to share 

best-practices from other arenas. This 

multicompetences ability can help 

you drive innovation in your application, 

delivering a competitive advantage 

through a holistic approach. 

WEBSITE 

https://www.aerotech.com/ 

https://www.attocube.com/en 

https://www.cedrat-technologies.com/en/ 

https://www.labmotionsystems.com/ 

http://www.madcitylabs.eu/ 

https://www.mksinst.com/b/newport 

https://npoint.com/ 

https://www.physikinstrumente.com/en/ 

https://piezomotor.com/ 

https://www.piezosystem.com/ 

https://piezoconcept-store.squarespace.com/ 

https://www.pro-lite.fr/ 

https://www.smaract.com/index-en 

https://www.tekceleo.fr/ 

https://www.thorlabs.com/ 


NEW PRODUCTS 

Optical frequency comb synthesizer 

Menlo Systems introduces 

the FC1500-LN nova , its latest 

optical frequency comb 

synthesizer model. The enhanced 

design of the laser 

oscillator results in significantly 

improved robustness 

against acoustical distortion 

and thermal drift. The major 

benefit of this novel design is a reduced free running linewidth 

of only 15 kHz. Owing to this leap in linewidth reduction, the 

FC1500-ULN nova has proven to support a frequency stability 

on the 10 -19 level within 1 s averaging time. 

www.menlosystems.com/products/optical-frequencycombs/fc1500-250-uln/ 

SWEPT LASER 

The Quantifi 

Photonics Laser 

2000 Series is a 

laboratory-grade 

O-band or C-band 

swept, tunable 

laser that can be operated as both a step-tuned or 

swept-wavelength laser source. With 0.01 dB power 

stability, 400 nm/s high-speed scan rate, and built-in 

synchronization trigger inputs and outputs, users can 

synchronize the laser sweep with other measurement 

tools such as optical power meters, spectrum analyzers, 

oscilloscopes, and more. 

www.quantifiphotonics.com/products/lasers-amplifiers/ 

matriq-swept-laser/ 

Dual Color Stimulated Raman Scattering 

The new deltaEmerald allows simultaneous Stimulated Raman 

Scattering (SRS) imaging of two vibrational bands with its dual color 

SRS (DC-SRS) scheme. Two Stokes pulses, separated by 85 cm -1 and 

modulated at different frequencies are overlapped with the tunable 

Pump pulse. Fully automated tuning, power control and temporal and 

spatial overlap of all three beams are given. Additionally a ~100 fs output 

at 1030 nm is provided for efficient SHG and two-photon imaging. 

www.ape-berlin.de/en/dc-srs/ 

LASER BEAM 

PROFILE ANALYZER 

The SLED 1000 Ophir® 

SP932U is a compact 

device designed and 

developed for analyzing 

the profile of laser 

beams in the UV, 

VIS, NIR and Nd:YAG 

wavelength ranges. It 

offers a wide dynamic 

range, high sensitivity 

and linearity, and high 

resolution. The SP932U profile analyzer offers a resolution 

of 2048 × 1536 pixels, a pixel spacing of 3.45 µm 

and a repetition rate of 24 Hz at full resolution. 

www.ophiropt.com/laser--measurement/beamprofilers/ 

High-Speed LCOS Spatial Light Modulator 

SLM-210 is the 

high speed model 

of SANTEC. 

SLM-210 features 

a significantly improved 

response 

speed. It is a high 

performance product 

which uses its second generation LCOS technology. 

Its response time of less than 10 ms is expected 

to contribute to the improvement of performances in 

optical applications such as wavefront correction, optical 

beam shaping for laser processing, biosensing and 

quantum computing. 

www.santec.com/en/products/components/slm/slm -210/

Photoniques 113

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